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>

priv/gettext/default.pot

@@ -1385,16 +1385,6 @@ msgstr ""
 msgid "From %{first} to %{last} (relevant years with membership data)"
 msgstr ""
 
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From MAIL_FROM_EMAIL"
-msgstr ""
-
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From MAIL_FROM_NAME"
-msgstr ""
-
 #: lib/mv_web/live/global_settings_live.ex
 #, elixir-autogen, elixir-format
 msgid "From OIDC_ADMIN_GROUP_NAME"
@@ -1430,31 +1420,11 @@ msgstr ""
 msgid "From OIDC_REDIRECT_URI"
 msgstr ""
 
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From SMTP_HOST"
-msgstr ""
-
 #: lib/mv_web/live/global_settings_live.ex
 #, elixir-autogen, elixir-format
 msgid "From SMTP_PASSWORD"
 msgstr ""
 
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From SMTP_PORT"
-msgstr ""
-
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From SMTP_SSL"
-msgstr ""
-
-#: lib/mv_web/live/global_settings_live.ex
-#, elixir-autogen, elixir-format
-msgid "From SMTP_USERNAME"
-msgstr ""
-
 #: lib/mv_web/live/global_settings_live.ex
 #, elixir-autogen, elixir-format
 msgid "From VEREINFACHT_API_KEY"
@@ -3917,3 +3887,13 @@ msgstr ""
 #, elixir-autogen, elixir-format
 msgid "join page URL in a new tab"
 msgstr ""
+
+#: lib/mv_web/live/global_settings_live.ex
+#, elixir-autogen, elixir-format
+msgid "SMTP environment configuration appears incomplete. Missing: %{keys}"
+msgstr ""
+
+#: lib/mv_web/live/global_settings_live.ex
+#, elixir-autogen, elixir-format
+msgid "SMTP is fully managed via environment variables. All SMTP fields are read-only."
+msgstr ""