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>

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Changed
+- **Clickable table row highlights** – The new hover/focus-visible row highlight behavior is now the CoreComponents default across clickable tables. Sticky-first-column tables keep zebra striping and show selection through the sticky-column accent stripe (checkboxes keep their default style).
+- **Members overview scrolling** – The members table scrollbar now scrolls inside the table container instead of moving with the full page.
 - **Join request display and settings workflow** – Improved join request rendering and related settings behavior in one cohesive update:
   - Join request fields now respect their configured field types in the details view.
   - Custom field labels in join request views were standardized.
@@ -17,6 +19,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - **Runtime ENV handling** – Empty or invalid environment variables (e.g. `SMTP_PORT=`, `PORT=`, `POOL_SIZE=`, `DATABASE_PORT=`) no longer cause `ArgumentError` at boot. Instead raises clear errors for required vars set but empty (e.g. DATABASE_HOST, PHX_HOST/DOMAIN, SECRET_KEY_BASE).
 - **PostgreSQL 18 Docker volume path** – Corrected the database volume path to match PostgreSQL 18 expectations.
+- **Association name ENV handling** – `ASSOCIATION_NAME` is now treated as source of truth; the field is read-only in Global Settings when managed via ENV.
+- **Association name consistency after updates** – Layout now prefers explicitly assigned `club_name` values to avoid stale cached values right after settings changes.
+- **SMTP ENV/UI source selection** – SMTP now follows a strict single-source policy: ENV-only when `SMTP_HOST` is set, otherwise Settings-only.
+- **SMTP settings UI in ENV mode** – SMTP fields are read-only, save action is hidden, and missing required ENV keys are shown as a warning.
 
 ### Dependency updates
 - Mix dependencies were updated.