feat: sidebar toggle shortcut + configurable keyboard shortcuts

[YapWH1208]

What

Adds a keyboard shortcut to open/close the sidebar, and makes all three of the plugin's keyboard shortcuts user-configurable.

• Sidebar toggle: default Ctrl/Cmd+B (Mod+B) flips the right panel from anywhere in the GUI. The chord yields to IME composition, to any handler that already claimed the event (defaultPrevented), to auto-repeat, and to the sidebar terminal (where Ctrl+B keeps its shell meaning: control char / tmux prefix).
• Configurable shortcuts: the Side card settings page gains a "Keyboard shortcuts" group with one text row per shortcut — toggle sidebar, save editor, git commit. Chords persist in SidebarPrefs.shortcuts (validated + canonicalized) and apply live (toggle listener, CodeMirror keymap compartment, GitView commit).
• Chord syntax: Mod+B, Ctrl+Shift+Enter, Cmd+S, … — Mod = Cmd on macOS, Ctrl elsewhere; at least one modifier required; named keys (Enter, Space, Tab, Escape, arrows, F1–F12, …) supported.
• Tooltips (toggle button, save button, git commit placeholder) show the live chord; README (zh/en) documents defaults and where to rebind them.

Why

The sidebar previously could only be toggled by clicking, and the plugin's hard-coded shortcuts were not discoverable/configurable. User request: "add shortcut to open/close the sidebar" plus configurability for all available shortcuts.

Design

docs/plans/2026-08-14-shortcuts-design.md (repo-convention design doc incl. deviation log, review round, acceptance and security records).

Tests

• New tests/shortcuts.spec.ts (34 tests): chord parse/canonical/display/match + CodeMirror key mapping matrix; toggle listener (session gating, terminal/.xterm protection, defaultPrevented/IME/repeat yields, pref-driven chords, platform quirk, disposer).
• tests/unit.spec.ts: parsePrefs shortcuts-map validation/canonicalization + special-key hygiene.
• tests/side-card-section-rows.spec.tsx: ShortcutRow interactive behavior (commit, invalid revert, Enter commit, failed-write remount).
• Existing suites updated for the new prefs field.
• pnpm typecheck ✓, pnpm build ✓, vitest: 441 passed; the only failures (22) are pre-existing node-pty spawn failures in smoke/agent-pty specs, reproduced identically on clean main (sandbox cannot spawn shells).

Review evidence

• Independent code review (isolated subagent): REQUEST-CHANGES → all findings fixed in 3b313a2 (macOS Cmd+Shift quirk, commit-chord preventDefault, live commit placeholder, repeat/no-session handling, Mod+Ctrl/Cmd rejection, stale-merge fix, +regression tests).
• Independent acceptance (isolated subagent): ACCEPT-WITH-NOTES — notes addressed (save tooltip live chord; no-input-guard documented as user-approved behavior; conflict detection explicitly out of scope).
• Independent security audit (isolated subagent): LOW-RISK, no exploitable items — hardening applied in 797552c (special-key hygiene + test, cached platform detection, browser-conflict hint copy).

Risk / migration

• No migration: shortcuts is an open map defaulting to {} (old settings documents resolve with defaults).
• Behavior note: the git-commit default chord is now Mod+Enter (Cmd+Enter on macOS; previously both Ctrl and Cmd+Enter worked there) — rebindable in settings.
• A user-configured chord can shadow browser defaults (e.g. Ctrl+W); the settings copy warns; conflict detection is intentionally out of scope (documented in the design doc).
• No security impact: keybinding feature only; no new routes or permissions.

Validation

typecheck ✓ · vitest 441 passed (22 pre-existing pty-spawn failures on clean main) · build ✓ · git diff --check ✓

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