feat(web): rebindable keyboard shortcuts and a Keybinds settings page - #470

🟢 Confidence 10/10 · safe to merge. The earlier per-row Reset defect is fixed and pinned by a scene test; keymap uniqueness, entry cleaning and binding paths are read and tested.
_{no findings · tests covered · risk low}

Adds rebindable shortcuts: defineKeymap replaces the hotkey registry and catalog, overrides are stored per device and resolved into Shared.keys, and a Keybinds page records, resets and unbinds them. The earlier per-row Reset finding is fixed, and the McpQueryError issues are untouched by this diff. Safe to merge.

• defineKeymap declares every action once and keeps rebindable keys unique in resolve
• Layout and channel hotkeys bind the user's resolved keys as a subscription dependency
• New /:org/my-settings/keybinds page records, refuses typing keys, resets and unbinds
• The ⌘/ dialog lists current keys and links to the settings page

Fixed since the last review

• ✅ F2 · A shortcut whose key another row took has no per-row Reset

Production impact

_{6fdd136 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}