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>

CODE_GUIDELINES.md

@@ -1277,7 +1277,8 @@ mix hex.outdated
 
 **SMTP configuration:**
 
-- SMTP can be configured via **ENV variables** (`SMTP_HOST`, `SMTP_PORT`, `SMTP_USERNAME`, `SMTP_PASSWORD`, `SMTP_PASSWORD_FILE`, `SMTP_SSL`) or via **Admin Settings** (database: `smtp_host`, `smtp_port`, `smtp_username`, `smtp_password`, `smtp_ssl`). ENV takes priority (same pattern as OIDC/Vereinfacht).
+- SMTP can be configured via **ENV variables** (`SMTP_HOST`, `SMTP_PORT`, `SMTP_USERNAME`, `SMTP_PASSWORD`, `SMTP_PASSWORD_FILE`, `SMTP_SSL`) or via **Admin Settings** (database: `smtp_host`, `smtp_port`, `smtp_username`, `smtp_password`, `smtp_ssl`).
+- **ENV-only policy:** If `SMTP_HOST` is set, SMTP is treated as environment-managed only. All SMTP fields in Settings are read-only, SMTP save action is hidden, and the UI shows a warning when required ENV values are missing (`SMTP_USERNAME`, and `SMTP_PASSWORD` or `SMTP_PASSWORD_FILE`). This keeps one source of truth for transport credentials and avoids mixed ENV/DB SMTP states.
 - **Sensitive settings in DB:** `smtp_password` and `oidc_client_secret` are excluded from the default read of the Setting resource; they are loaded only via explicit select when needed (e.g. `Mv.Config.smtp_password/0`, `Mv.Config.oidc_client_secret/0`). This avoids exposing secrets through `get_settings()`.
 - **Settings cache:** `Mv.Membership.get_settings/0` uses `Mv.Membership.SettingsCache` when the cache process is running (not in test). Cache has a short TTL and is invalidated on every settings update. This avoids repeated DB reads on hot paths (e.g. `RegistrationEnabled` validation, `Layouts.public_page`). In test, the cache is not started so all callers use `get_settings_uncached/0` in the test process (Ecto Sandbox).
 - **Join emails (domain → web):** The domain calls `Mv.Membership.JoinNotifier` (config `:join_notifier`, default `MvWeb.JoinNotifierImpl`) for sending join confirmation, already-member, and already-pending emails. This keeps the domain independent of the web layer; tests can override the notifier.