Split Ctrl+W: close-tab vs close-pane shortcuts

[mvbmir]

Summary

Follow-up to #18. Previously Ctrl+W closed the entire pane (including all its tabs), which is surgery most users don't want by default. This PR splits the bindings so the common case (close one tab) has the familiar shortcut and the aggressive one moves to a modifier.

• New shortcut: close_focused_tab — default Ctrl+W. Closes the active tab in the focused pane. If that was the last tab, the pane's existing on_empty callback fires and the pane is removed (and the workspace if it was the last pane).
• Remapped: close_focused_pane — default Ctrl+Alt+W (was Ctrl+W). Same behavior as before — closes the whole pane with all its tabs.
• Configurable: both appear in the keybindings editor, can be remapped or cleared by the user.
• Close-pane button tooltip already reads from the resolved shortcut config (pane_action_tooltip → shortcuts.tooltip_text), so it automatically reflects whichever binding is active, including user overrides.

Base: mvbmir/monitor-ghostty-actions (the redesign PR #18). Will rebase / base-change to main after #18 merges.

Follow-ups to consider before merging

• Search bar in the keybindings editor — 48 shortcuts and growing; a simple filter input would help a lot with finding and remapping. Not blocking this PR but would be a natural companion change.
• Allow fully unbinding a shortcut — verify the editor supports clearing a binding entirely for users who'd rather not have a specific action mapped at all. If it doesn't, add explicit UX for it.

Notes for reviewers

• Users with shortcuts.json overrides for close_focused_pane keep their existing binding. Users who want Ctrl+W to still close the whole pane can restore that by binding close_focused_pane to <Ctrl>w in the editor (and rebinding close_focused_tab elsewhere or clearing it).
• Middle-click-to-close-tab is unchanged (that lives in Redesign top bar + sidebar, add settings (#5) #18, not here).

Test plan

• cargo fmt clean
• cargo clippy --release -- -D warnings clean
• cargo test — all 154 unit tests pass (definition count updated 47 → 48)
• Manual: Ctrl+W closes a single tab, pane stays when 2+ tabs present
• Manual: Ctrl+W on a pane's last tab closes the pane
• Manual: Ctrl+W on the workspace's last pane's last tab closes the workspace
• Manual: Ctrl+Alt+W closes the pane regardless of tab count
• Manual: close-pane button tooltip reads "Close pane (Ctrl+Alt+W)" by default
• Manual: keybindings editor shows both Close Focused Tab and Close Focused Pane; both can be remapped and cleared