fix(ui): improve hover contrast for ghost buttons and the settings menu

[cashcon57]

Describe Your Changes

Follow-on contrast polish to #106 / #140 (which addressed selected states). Those
PRs left the hover states using absolute bg-accent / bg-secondary overlays,
which collapse against light backgrounds (both tokens are ≈0.97 in light, nearly
identical to the surfaces they sit on) — so hover was barely visible, especially in
light mode.

Two fixes, both switching hover to a background-relative bg-foreground/10
overlay so it's legible on any surface in both themes, and stays a step below the
selected state's bg-foreground/20 (hover < selected):

• Ghost buttons (components/ui/button.tsx) — the ghost variant's
  hover:bg-accent was the faint hover on every ghost button (icon toolbars like
  the message copy/edit/delete/regenerate actions, header icons, menus, …). This is
  a shared design-system change — it affects all variant="ghost" buttons app-wide
  (~80 usages). outline is intentionally left as-is (it has a border).
• Settings menu (SettingsMenu.tsx) — the category links used hover:bg-secondary
  while the model-provider rows used hover:bg-secondary/60, so they didn't match
  and both were faint in light. Unified to the same hover:bg-foreground/10.

Styling-only change (no logic, no new dependencies), verified in a local dev build.

Before / After

Fixes Issues

No linked issue — UX/accessibility polish (follow-on to the #107 contrast work).

Self Checklist

• Added relevant comments, esp in complex areas — n/a (CSS-class-only change)
• Updated docs (for bug fixes / features) — n/a
• Created issues for follow-up changes or refactoring needed — n/a

[cashcon57]

Folded into #140, which now covers both the selected and hover contrast fixes as one comprehensive pass.