fix(sidebar): fix @for layout, empty secondary divider, dark mode icons, active state (#608)

[giulio-leone]

Fixes #608

Problems fixed

1. @for loop breaks sidebar layout
Custom <it-list-item> elements have display: inline by default, breaking the <ul> list layout. Added display: contents to :host(:not(li)) in list-item.component.scss so the inner <li> acts as a direct layout child.

2. Secondary divider visible when no secondary links
The .linklist-secondary wrapper always renders even when empty. Added CSS rule :not(:has(*)) to hide it when no secondary content is projected.

3. Dark mode icon colors
Icons inside the sidebar don't inherit the white text color in dark mode and turn blue on hover. Added ::ng-deep rules under :host(.theme-dark) to force icon color inheritance.

4. Active state styling
Active classes are correctly applied by the component; the visual issue was caused by Problem 1 (broken layout). Fixed by the display: contents solution.

Files changed

• list-item.component.scss — Added :host(:not(li)) { display: contents; }
• sidebar.component.scss — NEW: CSS for empty secondary + dark mode icons
• sidebar.component.ts — Added styleUrls
• sidebar.component.spec.ts — 15 new tests covering all 4 problems

Verification

• ✅ 125/125 unit tests passing (double gate)
• ✅ 0 lint errors (8 pre-existing warnings)
• ✅ Lint-staged + commitlint passed

⚠️ This reopens #636 which was accidentally closed due to fork deletion.