feat: add Gmail-style keyboard navigation to spec and plan

Task 14: keyboard.js with focus cursor, multi-select, and action
keys (j/k navigate, x/Space select, # delete, e archive, Shift+i/u
read/unread, v move). New keyboardMultiSelect setting and CSS.

docs/superpowers/specs/2026-04-23-outlook-relook-design.md

@@ -175,6 +175,40 @@ Changing the density preset sets all individual density toggles to the preset's
 | Auto-resize compose window | Boolean | On |
 | Throttle desktop notifications | Boolean | Off |
 
+### Keyboard Navigation
+
+Gmail-style keyboard navigation and multi-select for the message list. OWA lacks the ability to select multiple messages and act on them purely from the keyboard — this is the single biggest functionality gap vs Gmail.
+
+| Toggle | Type | Default |
+|--------|------|---------|
+| Keyboard multi-select mode | Boolean | On |
+
+**Key bindings (active when message list is focused, not during compose):**
+
+| Key | Action |
+|-----|--------|
+| `j` / `Down Arrow` | Move focus to next message |
+| `k` / `Up Arrow` | Move focus to previous message |
+| `x` or `Space` | Toggle select on focused message (multi-select) |
+| `Shift+j` / `Shift+Down` | Select and move to next (extend selection) |
+| `Shift+k` / `Shift+Up` | Select and move to previous (extend selection) |
+| `#` | Delete selected message(s) |
+| `e` | Archive selected message(s) |
+| `Shift+i` | Mark selected as read |
+| `Shift+u` | Mark selected as unread |
+| `v` | Move selected (open OWA's move-to-folder dialog) |
+| `Escape` | Deselect all |
+| `Enter` / `o` | Open focused message in reading pane |
+
+**Implementation approach:**
+
+- The extension manages its own "focus cursor" on the message list, visually distinguished from OWA's native selection (e.g., a left-border highlight or subtle background tint)
+- A Set tracks "checked" (multi-selected) message elements, visually marked with a checkbox indicator or distinct background
+- Action keys (`#`, `e`, `Shift+i`, etc.) iterate over checked messages and dispatch OWA's native actions for each (via toolbar button clicks, context menu triggers, or keyboard shortcut forwarding)
+- If no messages are checked, actions apply to the currently focused message (single-select behavior, matching Gmail)
+- All keyboard handling is suppressed when a compose window, search bar, or dialog has focus — detected via `activeElement` checks
+- A new file `content/keyboard.js` encapsulates all keyboard navigation logic, separate from `behavior.js`
+
 ### Quick Actions (injected UI)
 
 | Toggle | Type | Default |
@@ -202,6 +236,7 @@ outlook-relook/
 │   ├── observer.js            # MutationObserver logic, element suppression
 │   ├── selectors.js           # Selector registry (logical name -> strategies)
 │   ├── behavior.js            # JS behavior tweaks
+│   ├── keyboard.js            # Gmail-style keyboard navigation & multi-select
 │   └── injector.js            # DOM injection (mark-all-read, folder jump)
 ├── themes/
 │   ├── base.css               # Shared density/spacing/hiding overrides