fix(ui): improve selected and hover contrast across the app

[cashcon57]

Describe Your Changes

A pass over the app's low-contrast selected/hover states. Several used an absolute
bg-secondary / bg-accent overlay, which collapses against light backgrounds
(both ≈0.97 in light, nearly identical to the surfaces they sit on) — so the
state was hard to see, especially in light mode, and in places hover read as strong
as (or stronger than) the selection.

Everything moves to a background-relative foreground overlay so it stays
legible on any surface in both themes, with hover (/10) a clear step below
selected (/20).

Design decisions

• Relative overlay, not an absolute token. bg-secondary/bg-accent are ≈0.97
  in light — the same lightness as the surfaces behind them, so they vanish. A
  foreground overlay is a fixed delta from whatever is behind it, so it holds up
  on any surface and flips correctly between themes.
• /10 hover, /20 selected. Two fixed steps keep hover clearly below selected;
  before, several menus had them equal (or hover stronger).
• Fixed at the source for shared components. The ghost button variant and the
  base dropdown-menu item states are each changed once, so every ghost button (~80)
  and every dropdown inherits it rather than being patched per call site.
• outline variant left alone — it has a border, so its hover already reads fine.
• Styling only — no logic, state, or dependency changes.

Selected states → bg-foreground/20

• Model selector (DropdownModelProvider), search dialog (SearchDialog), Hermes
  Agent / Claude Code model rows, dropdown control (DropdownControl), assistant
  switcher (SamplerPopover). The search dialog's selected row was previously
  lighter than its hover — now clearly stronger.
• Hub sort dropdown (hub/index) and default-assistant picker (settings/assistant)
  — the selected item used bg-secondary (0.97), the exact same lightness as the
  menu hover, so selected and hovered items were indistinguishable.

Hover states → bg-foreground/10

• ghost button variant (components/ui/button.tsx) — hover:bg-accent was the
  faint hover on every ghost button (message copy/edit/delete/regenerate, header
  icons, menus, …). Shared design-system change — affects all variant="ghost"
  buttons app-wide (~80 usages). outline is left as-is (it has a border).
• Menu item highlight (components/ui/dropdown-menu.tsx) — the base
  focus:bg-accent / data-[state=open]:bg-accent states. Also a shared
  design-system change — affects every dropdown menu app-wide, and makes menu
  items hover consistently with ghost buttons.
• Settings menu (SettingsMenu) — category links used hover:bg-secondary while
  provider rows used hover:bg-secondary/60 (mismatched and faint in light); unified.
• Model edit / delete trigger icons (EditModel, DeleteModel) — these had no
  hover feedback at all; added.
• Download panel pause / resume / cancel controls (DownloadManegement) — these were
  variant="secondary" on a bg-secondary row, so the button was the same shade as
  its surface and hover:bg-secondary/80 only faded it further. Switched to ghost
  so they share the hover overlay and read on the row.
• A final sweep of the remaining faint row/icon hovers (hover:bg-{muted,secondary}
  at assorted opacities): hub model variant rows (HubModelCard), project file rows
  (ProjectFiles), security config rows (SecurityConfigDialog), assistant cards,
  and the small utility icon buttons in AddEditMCPServer, AddEditCustomCliDialog,
  AudioPlayer, Capabilities, ApiKeyInput, AttachmentChip, hermes-agent.
  These had drifted across several ad-hoc values; they now share the one hover overlay.

Companion to #106 (sidebar/nav/settings selected states, already merged).
Styling-only — no logic, no new dependencies — verified in a local dev build.

Before / After

Light mode (where the change is most visible) — stock v1.1.119 vs. this branch:

Model selector — row hover

Hub sort dropdown — selected vs. hover (were the same shade before)

Model edit button — hover (no hover before)

Search (⌘K) — highlighted result

Settings menu — hover vs. selected

Fixes Issues

• Closes bug: low-contrast selected/highlighted states (most visible in light mode) #107

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