Add terminal big mode, keyboard shortcuts menu, and UX refinements

- Reduce mobile terminal widths by 2 chars (portrait 42x24, landscape 86x24)
- Add "Big mode" for mobile: desktop sizing (120x36) at 70% zoom
- Click zoom percentage to reset to 100%
- Add keyboard shortcuts submenu in session settings
- Update PRD with all terminal features and documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/src/spiceflow/adapters/tmux.clj

@@ -339,10 +339,10 @@
 
 ;; Screen size presets for different device orientations
 (def ^:private screen-sizes
-  {:fullscreen {:width 260 :height 48}
-   :desktop    {:width 120 :height 48}
-   :landscape  {:width 80  :height 24}
-   :portrait   {:width 40  :height 24}})
+  {:fullscreen {:width 260 :height 36}
+   :desktop    {:width 120 :height 36}
+   :landscape  {:width 86  :height 24}
+   :portrait   {:width 42  :height 24}})
 
 (defn resize-session
   "Resize a tmux session window to a preset size.