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>

docs/smtp-configuration-concept.md

@@ -25,7 +25,11 @@ Enable configurable SMTP for sending transactional emails (join confirmation, us
 | ENV      | 1        | Production, Docker, 12-factor     |
 | Settings | 2        | Admin UI, dev without ENV          |
 
-When an ENV variable is set, the corresponding Settings field is read-only in the UI (with hint "Set by environment").
+When `SMTP_HOST` is set, SMTP runs in **ENV-only mode**:
+- all SMTP fields in Settings are read-only,
+- saving SMTP settings in the UI is disabled,
+- and the UI shows a warning block if required SMTP ENV values are missing.
+- the UI displays the effective ENV-driven SMTP values in disabled fields so admins can verify what is active.
 
 ---
 
@@ -63,6 +67,14 @@ Support **SMTP_PASSWORD_FILE** (path to file containing the password), same patt
   - Show a warning in the Settings UI.
   - Delivery attempts silently fall back to the Local adapter (no crash).
 
+### 6.1 Behaviour in ENV-only mode (`SMTP_HOST` set)
+
+- The SMTP source of truth is environment variables only.
+- The UI does not allow editing SMTP fields in this mode.
+- The Settings page shows a warning block when required values are missing:
+  - `SMTP_USERNAME`
+  - `SMTP_PASSWORD` or `SMTP_PASSWORD_FILE`
+
 ---
 
 ## 7. Test Email (Settings UI)