Add letter-prefix tile labels mode

[peterp]

Stacked on top of #15 — review/merge that first; this branch will retarget to main automatically once it lands.

Summary

Adds a new Tile labels setting that lets the user pick between the existing 1-9 numeric picks and a new letters mode where every tile is labelled with a 2-char prefix derived from its app name (gc Google Chrome, wa WhatsApp, cu Cursor, cc Claude Code). Type the prefix to pick + activate the window.

Behavior

• Prefix algorithm: tokenize the app name on whitespace and camelCase boundaries (WhatsApp → [Whats, App], VSCodium → [VS, Codium]). 2+ tokens → first letter of first two; 1 token → first two letters.
• Same-app duplicates: keep the first letter, pick the second from a home-row pool (j k l f d s a g h) — second Chrome window gets gj, third gk, etc.
• Cross-app collisions: extend to 3 chars from the app name (Calendar vs Camera → ca vs cam).
• Sticky assignments: prefixes persist for the lifetime of the process. Closing the original gc Chrome window does not reshuffle the second window's gj prefix.

Input

• A typeahead buffer matches the prefix as you type. The matched portion on each tile's chip renders in yellow; tiles whose prefix does not match the buffer dim to 30%. A unique full match auto-picks.
• delete (backspace) pops the last char.
• esc clears the buffer; pressing it again with an empty buffer dismisses.
• wasd movement, 1-9 picks, and ⌃+letter app-jump are suppressed in letters mode. Arrows still move; ⌘+arrow still swaps; search and tag bindings are unaffected.

Files

• LabelAssigner.swift (new) — tokenizer + sticky assignment.
• Tile.swift — setLabel(_:matchPrefix:) replaces setNumber; chip widens for multi-char strings; matched portion rendered via NSAttributedString on the existing CATextLayer.
• Config.swift, SettingsWindow.swift — new tilePicks enum + segmented picker in Settings.
• Overlay.swift, OverlayView.swift — typeahead buffer, mode-aware label render, keymap bypass for letters/digits/backspace when letters mode is active.

Test plan

• Settings → Tile labels → Letters. Reopen overlay; tiles show 2-char chips.
• Type a letter — matching tiles' prefix highlights yellow on the matched chars; non-matching tiles dim. Type the next char to narrow down. Unique match auto-picks + activates.
• Open two Chrome windows — first is gc, second is gj. Close the first; the second stays gj.
• Calendar + Camera both want ca → one gets ca, the other cam.
• Backspace pops a char; esc clears buffer; second esc dismisses.
• In letters mode, wasd, 1-9, and ⌃+letter are no-ops; arrows still move; ⌘+arrow still swaps.
• Switch back to Numbers mode → tiles show 1-9; wasd, digit picks, and ⌃+letter work again.