fix(desktop): space out find-bar buttons and anchor it below the titlebar

[tyfcontent]

What

Two related fixes to the find-in-page bar (Ctrl+F / ⌘F):

1. Anchor the bar below the titlebar. The top-[...] calc used a 0px fallback for --titlebar-height, but that variable is unset at the find bar's mount point (the contrib shell zeroes it and hardcodes the titlebar to h-[34px]), so the bar rendered at ~8px — inside the titlebar, over the window controls. Use the same 34px fallback as the sibling overlays (notifications.tsx, floating-hud.ts).

2. Space out and enlarge the controls. Buttons go from h-5 w-5 (20px) to h-7 w-7 (28px) with matching icon glyph bumps, and the container gap goes from gap-1 to gap-2 (plus py-1.5), so the previous/next/close buttons are no longer crammed together and mis-clicked.

Test plan

• npx vitest run src/components/find-bar.test.tsx — 48/48 passing
• npx eslint src/components/find-bar.tsx — clean
• Manual: open the app, press Ctrl+F, type a query — bar sits below the titlebar with comfortable button spacing; Escape / Enter / ⌘G still work.

Changes

apps/desktop/src/components/find-bar.tsx

[teknium1]

Salvaged into #86840 (merged) — your spacing/hit-target polish landed with your authorship preserved via cherry-pick. The positioning half (34px fallback) was dropped only because it had already landed via #86746/#80244. Thanks @tyfcontent!