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>

test/mv_web/member_live/index_test.exs

@@ -78,6 +78,37 @@ defmodule MvWeb.MemberLive.IndexTest do
       assert html =~ "lg:top-0"
       assert html =~ "bg-base-100"
     end
+
+    test "members page does not nest a second overflow wrapper inside members-table-scroll", %{
+      conn: conn
+    } do
+      conn = conn_with_oidc_user(conn)
+      {:ok, _view, html} = live(conn, ~p"/members")
+
+      assert html =~ ~s(id="members-keyboard")
+      assert html =~ ~s(class="overflow-visible")
+      refute html =~ ~s(id="members-keyboard" class="overflow-x-auto")
+      refute html =~ ~s(id="members-keyboard" class="overflow-auto")
+    end
+
+    test "members table keeps checkbox column sticky while horizontally scrolling", %{conn: conn} do
+      system_actor = SystemActor.get_system_actor()
+
+      {:ok, _member} =
+        Membership.create_member(
+          %{first_name: "Sticky", last_name: "Column", email: "sticky-column@example.com"},
+          actor: system_actor
+        )
+
+      conn = conn_with_oidc_user(conn)
+      {:ok, _view, html} = live(conn, ~p"/members")
+
+      # Contract: first column (select-all header + row checkbox cells) is sticky on the left
+      assert html =~ "left-0"
+      assert html =~ "sticky"
+      assert html =~ "z-30"
+      assert html =~ "z-20"
+    end
   end
 
   describe "translations" do
@@ -339,10 +370,12 @@ defmodule MvWeb.MemberLive.IndexTest do
     assert_redirect(view, ~p"/members/#{member}")
   end
 
-  describe "table row outline (hover and selected)" do
+  describe "table row highlight (hover and selected)" do
     @describetag :ui
 
-    test "clickable rows have hover and focus-within ring classes", %{conn: conn} do
+    test "clickable rows with sticky first column use hover/focus background highlight", %{
+      conn: conn
+    } do
       system_actor = SystemActor.get_system_actor()
 
       {:ok, _member} =
@@ -354,10 +387,9 @@ defmodule MvWeb.MemberLive.IndexTest do
       conn = conn_with_oidc_user(conn)
       {:ok, _view, html} = live(conn, "/members")
 
-      # CoreComponents table adds hover and focus-within ring when row_click is set
-      assert html =~ "hover:ring-2"
-      assert html =~ "focus-within:ring-2"
-      assert html =~ "hover:ring-base-content/10"
+      # Sticky-first-column tables: hover/focus fills live in CSS; wrapper is marked for tests.
+      assert html =~ ~s(data-sticky-first-col-rows="true")
+      refute html =~ "hover:ring-2"
     end
 
     test "selected outline only from checkbox selection, not from highlight param", %{conn: conn} do