feat: resize the ui sidebar with C-a Left/Right

[kylecarbs]

What

C-a Left/C-a Right starts a sidebar resize in boo ui, mirroring the C-a Up/C-a Down browse:

• Bare Left/Right keep adjusting the width one column per press while the resize is active.
• Enter completes the resize and keeps the width.
• Esc cancels and restores the width from before the first arrow.
• The bottom row shows left/right resize enter done esc cancel while active, and the armed-prefix bar gained lt/rt resize.

The viewport and the focused session's pty resize live with every adjustment. A completed resize is remembered and reapplied (clamped to [8, cols - 12], like Layout.init's narrow-terminal floor) when the terminal itself resizes.

How

• The input parser now intercepts plain ESC [ C / ESC [ D exactly like up/down arrows; InputEvent.Arrow carries a four-way dir enum instead of an up bool. Modified arrows (ESC [ 1;5 C) still pass through untouched.
• Bare side arrows forward to the focused application whenever no resize is active, preserving existing behavior (the UI never mirrors DECCKM outward, so apps always receive plain CSI arrows).
• Resize and browse are mutually exclusive: starting one ends the other; prefix commands and mouse actions commit an active resize.

Validation

• Unit tests: side arrows parse (bare and prefixed), clamp bounds.
• Integration test drives the full flow through real ptys: grow, bare-arrow adjust, Esc restore, shrink, Enter commit, and bare arrows forwarding again afterwards, asserting the session pty size through peek --json at every step.
• The keybind-bar test widened from 120 to 132 columns for the longer bar.
• zig build test-all (Debug and ReleaseSafe): 120/120 pass.

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