fix(ui): recognize encoded arrows so C-a Up/Down/Left/Right works again

[kylecarbs]

Problem

After v0.5.16, pressing C-a then an arrow key no longer browses the sidebar (C-a Up/Down) or resizes it (C-a Left/Right) when a kitty-protocol app is focused.

Root cause

v0.5.16 mirrors the focused app's keyboard protocols onto the real terminal. When the app enables report event types (kitty flag 2, which Claude Code and most modern TUIs set), the terminal then encodes even an unmodified arrow press in the functional form ESC [ 1;1:1 A instead of the legacy ESC [ A (confirmed against ghostty's own encodeSpecial: any sequence with an event type emits \x1b[1;{mods}:{event}{ABCD}).

The UI input parser only recognized the bare ESC [ A form, so after the C-a prefix the encoded arrow diverged from the hold grammar and was flushed to the session — no browse/resize. Before v0.5.16 the terminal stayed in legacy mode, so arrows were ESC [ A and it worked.

Fix

• The parser now accepts the functional cursor-key grammar ESC [ 1 ; mods [: event] A/B/C/D (its hold grammar accepts A–D after a digit body).
• An unmodified press or repeat becomes an arrow event (drives browse/resize). A modified arrow (e.g. Ctrl+Left word motion) or a release event is the application's input and is forwarded verbatim, never intercepted.
• Arrow events now carry their original bytes; arrows forwarded to the focused app keep the exact encoding the terminal produced rather than a downgraded legacy arrow.

Testing

• 3 new parser unit tests: functional press/repeat navigate (prefixed and bare), original-byte forwarding, modified/release passthrough.
• 1 new PTY integration test: a kitty app focused, C-a + report-events Down arms sidebar browse without leaking into the session.
• zig build test-all (116 unit + 67 integration) passes, integration re-run 3x for flakes; zig fmt --check clean.

Notes

• SS3 cursor keys (ESC O A, application-cursor-keys mode) are intentionally not handled: boo ui never puts the real terminal into DECCKM, so that encoding cannot occur there. boo at forwards arrows to the app verbatim and is unaffected.
• Modified arrows and releases reaching the app verbatim preserves Ctrl+arrow word motion and report-events press/release pairing.

This PR was generated by Coder Agents on behalf of @kylecarbs.