Add first-letter app-jump in the overlay

[peterp]

While the overlay is up, hold ⌃ (Control) + an app's first letter to jump selection to the next tile owned by that app. Repeat ⌃+letter to cycle through matches.

• Match: `tile.scWindow.owningApplication?.applicationName`, lowercased prefix
• Order: from current+1 to end, then wraps from 0
• Reset: `lastLetterJump` clears on overlay hide so each show starts fresh
• Modifier: ⌃ chosen because no default bindings use it; bare wasd / arrow keys keep moving as before
• Disabled: silent no-op (same as any unmapped key today)

Toggle in Settings → "First-letter app jump" (default on, persisted as `letterJump` in config.json).

Credit to @plyght — concept from #1, with a Control gate added to avoid colliding with wasd movement bindings.

Test plan

• `./build-app.sh && open cmdcmd.app`
• ⌘⌘ overlay up → press `s` → selection moves down (wasd movement still works)
• Press ⌃+s → selection jumps to a Safari/Slack/Spotify tile
• Press ⌃+s again → selection cycles to the next matching app
• ⌃+letter with no matching app → no movement
• Disable in Settings → ⌃+letter is a no-op
• Dismiss overlay, reopen, ⌃+s — starts cycle from index 0 again

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