Improve member view table behavior+style, fix config settings (#493)

## Description of the implemented changes
The changes were:
- [x] Bugfixing
- [x] New Feature
- [ ] Breaking Change
- [x] Refactoring

This PR standardizes interactive table behavior and improves settings robustness.
It makes the new hover/focus-visible row highlight the default for clickable tables, keeps sticky first-column behavior configurable (and optimized for member selection UX), and tightens SMTP source-of-truth handling so ENV-based and UI-based configuration do not conflict.

## What has been changed?
- Refactored `CoreComponents.table` to expose interaction state via `data-row-interactive` and moved default row hover/focus styling to CSS.
- Made the new row highlight behavior (`hover` + `:has(:focus-visible)`) the default for clickable zebra tables.
- Kept sticky-first-column as an explicit table option and preserved sticky-specific selection accent behavior.
- Updated member overview table usage to the sticky-first-column mode and refined scrolling behavior (table scrollbar within container, not page-coupled).
- Adjusted table-related tests to validate the new interaction contract (attribute/CSS-driven behavior instead of legacy ring classes).
- Improved SMTP config handling:
  - clearer ENV-vs-Settings behavior (ENV-only mode when host env is set),
  - read-only and warning behavior in global settings UI when required env keys are missing,
  - updated related config/tests/docs.
- Updated docs and changelog (`CHANGELOG.md`, `DESIGN_GUIDELINES.md`, `CODE_GUIDELINES.md`, SMTP concept docs).
- Updated gettext catalogs (`default.pot`, `en`, `de`) for new/changed UI strings.

## Definition of Done
### Code Quality
- [x] No new technical depths
- [x] Linting passed
- [x] Documentation is added were needed

### Accessibility
- [x] New elements are properly defined with html-tags
- [x] Colour contrast follows WCAG criteria
- [x] Aria labels are added when needed
- [x] Everything is accessible by keyboard
- [x] Tab-Order is comprehensible
- [x] All interactive elements have a visible focus

### Testing
- [x] Tests for new code are written
- [x] All tests pass
- [ ] axe-core dev tools show no critical or major issues

## Additional Notes
- Branch includes 4 commits:
  - `fix: make sure smtp can be set either via env or ui`
  - `fix: make horizontal scrollbars sticky to bottom`
  - `docs: update changelog`
  - `feat: make checkbox column in member view sticky`
- Full fast suite passed (`mix test --exclude slow --exclude ui`): 2017 tests, 0 failures (plus expected non-failing warning logs in test output).
- Reviewer focus areas:
  1. **Cross-table UX consistency** after moving row interaction styling to component/CSS contract.
  2. **Sticky table behavior** (selection accent stripe, zebra background, keyboard focus visibility).
  3. **SMTP precedence and UI constraints** in global settings when ENV mode is active.
  4. **Regression risk in tests** that previously asserted ring-based row classes.
- No breaking API changes expected; behavior change is primarily visual/interaction-level and intentional.

Reviewed-on: #493
Co-authored-by: Simon <s.thiessen@local-it.org>
Co-committed-by: Simon <s.thiessen@local-it.org>

lib/mv_web/components/core_components.ex

@@ -938,6 +938,16 @@ defmodule MvWeb.CoreComponents do
     doc:
       "when true, thead th get lg:sticky lg:top-0 bg-base-100 z-10 for use inside a scroll container on desktop"
 
+  attr :wrapper_overflow_class, :string,
+    default: "overflow-x-auto",
+    doc:
+      "overflow class for the table wrapper; set to overflow-visible when outer container owns scrolling"
+
+  attr :sticky_first_col, :boolean,
+    default: false,
+    doc:
+      "when true, first header/body column gets sticky left positioning to keep selection controls visible"
+
   slot :col, required: true do
     attr :label, :string
     attr :class, :string
@@ -974,15 +984,19 @@ defmodule MvWeb.CoreComponents do
     ~H"""
     <div
       id={@row_click && "#{@id}-keyboard"}
-      class="overflow-auto"
+      class={@wrapper_overflow_class}
+      data-sticky-first-col-rows={@sticky_first_col && "true"}
       phx-hook={@row_click && "TableRowKeydown"}
     >
       <table class="table table-zebra">
         <thead>
           <tr>
             <th
-              :for={col <- @col}
-              class={table_th_class(col, @sticky_header)}
+              :for={{col, col_idx} <- Enum.with_index(@col)}
+              class={[
+                table_th_class(col, @sticky_header),
+                @sticky_first_col && col_idx == 0 && "sticky left-0 z-30 bg-base-100"
+              ]}
               aria-sort={table_th_aria_sort(col, @sort_field, @sort_order)}
             >
               {col[:label]}
@@ -1006,7 +1020,13 @@ defmodule MvWeb.CoreComponents do
           <tr
             :for={row <- @rows}
             id={@row_id && @row_id.(row)}
-            class={table_row_tr_class(@row_click, table_row_selected?(assigns, row))}
+            class={[
+              table_row_tr_class(
+                table_row_selected?(assigns, row),
+                @sticky_first_col
+              )
+            ]}
+            data-row-interactive={@row_click && "true"}
             data-selected={table_row_selected?(assigns, row) && "true"}
             title={@row_click && @row_tooltip}
           >
@@ -1026,6 +1046,13 @@ defmodule MvWeb.CoreComponents do
                 has_click = col[:col_click] || @row_click
                 classes = ["max-w-xs"]
 
+                classes =
+                  if @sticky_first_col && col_idx == 0 do
+                    ["sticky-first-col-cell sticky left-0 z-20" | classes]
+                  else
+                    classes
+                  end
+
                 classes =
                   if col_class == nil || (col_class && !String.contains?(col_class, "text-center")) do
                     ["truncate" | classes]
@@ -1040,7 +1067,7 @@ defmodule MvWeb.CoreComponents do
                     classes
                   end
 
-                # WCAG: no focus ring on the cell itself; row shows focus via focus-within
+                # WCAG: no focus ring on the cell itself; sticky zebra rows show keyboard focus via CSS :has(:focus-visible)
                 classes =
                   if @row_click && @first_row_click_col_idx == col_idx do
                     [
@@ -1111,30 +1138,11 @@ defmodule MvWeb.CoreComponents do
     end
   end
 
-  # Returns CSS classes for table row: hover/focus-within outline when row_click is set,
-  # and stronger selected outline when selected (WCAG: not color-only).
-  # Hover/focus-within are omitted for the selected row so the selected ring stays visible.
-  defp table_row_tr_class(row_click, selected?) do
-    has_row_click? = not is_nil(row_click)
-    base = []
-
-    base =
-      if has_row_click? and not selected?,
-        do:
-          base ++
-            [
-              "hover:ring-2",
-              "hover:ring-inset",
-              "hover:ring-base-content/10",
-              "focus-within:ring-2",
-              "focus-within:ring-inset",
-              "focus-within:ring-base-content/10"
-            ],
-        else: base
-
-    base = if selected?, do: base ++ ["ring-2", "ring-inset", "ring-primary"], else: base
-    Enum.join(base, " ")
-  end
+  # Returns CSS classes for table row selection styles.
+  # Hover/focus row highlighting is CSS-driven via [data-row-interactive] selectors in app.css.
+  # Sticky-first-column zebra tables use CSS accents and omit selected row ring classes.
+  defp table_row_tr_class(true, false), do: "ring-2 ring-inset ring-primary"
+  defp table_row_tr_class(_, _), do: ""
 
   defp table_th_aria_sort(col, sort_field, sort_order) do
     col_sort = Map.get(col, :sort_field)