Polish CustomTooltipDirective styling + consolidate theme-toggle off native title

Context

While adding tooltips to icon-only buttons in the page-header (PR #5345 follow-up branch feature/ui-missing-tooltips), two facts surfaced:

1. Two tooltip mechanisms in tree — CustomTooltipDirective (selector [matTooltip], ~36 .html files use it) and a single native HTML title="" on the theme-toggle button. The theme-toggle case is the only native holdout in the page chrome.
2. CustomTooltipDirective's current visual style is heavier than the OS-native equivalent — 12px font, 8px×12px padding, opaque #333 background. The native browser tooltip on the theme-toggle is visibly lighter and more compact, which reads better against both light and dark page backgrounds.

We picked the custom directive over native for three reasons that should stay: 250ms show delay (vs ~1.5s native), consistent OS/browser appearance, and themeability under dark/light. The fix is to polish the directive's look — not switch mechanisms.

Scope

• src/frontend/packages/core/src/shared/components/custom-tooltip/custom-tooltip.directive.ts
  • Font size 12px → 11px
  • Padding 8px×12px → 4px×8px
  • Background #333 → semi-transparent black (e.g. rgba(0,0,0,0.78)) so it doesn't render heavy on light page backgrounds
  • Consider reading a CSS custom property for the background so the value can vary across light/dark themes via the existing theme variables (cf. theme-toggle for a peer that already swaps style by mode)
• src/frontend/packages/core/src/shared/components/theme-toggle/theme-toggle.component.html
  • Replace title=\"Toggle theme ({{ getLabel() }})\" with matTooltip=\"Toggle theme ({{ getLabel() }})\"
  • Add CustomTooltipDirective to the component's imports[]
  • Result: a single mechanism handles all tooltips across stratos

Why deferred

The team has a hard sequencing pin on V2 retirement + remaining @ngrx removal waves landing first. This is small visual polish on top of a working mechanism — not blocking any user. Picking it up mid-V2-or-ngrx work risks dragging attention off the load-bearing migration.

Acceptance

• All 36 (+ theme-toggle = 37) tooltip sites render with the new lighter style
• Native browser tooltip no longer used anywhere in the page chrome
• Visual parity verified across both light and dark themes
• No regression in show delay / positioning / auto-flip behavior

Start with src/frontend/packages/core/src/shared/components/custom-tooltip/custom-tooltip.directive.ts and inspect its current styling, then read theme-toggle.component.html and its imports. Update the directive’s visual treatment and replace the theme-toggle native title with matTooltip using CustomTooltipDirective. Done means all 37 page-chrome tooltip sites use the lighter style, both themes look consistent, and delay, positioning, auto-flip, and native-title removal meet the acceptance criteria.