feat(shortcuts): add dedicated previous and next workspace actions #799

Problem

User feedback asks for separate keyboard navigation between panes and between workspaces.

In v0.4.6, Previous/Next pane (Mod+Shift+Up/Down) walks the flattened pane roster, so it may require many presses to leave a workspace with many panes. The command palette now has Left/Right workspace navigation, but that requires opening the palette. There is no dedicated global Previous/Next workspace action in the released or current main shortcut registry.

Proposal

Expose Previous workspace and Next workspace as configurable app actions, usable directly from Terminal and Chat.

Define and test:

• Navigation follows the selected PC's existing workspace order, including the handling of linked worktrees, and wraps consistently.
• The destination restores its last-viewed/focused valid pane using the app's existing workspace selection behavior.
• Switching retains each pane's lens and unsent draft, with no terminal keystroke leakage.
• Bindings obey the existing Mod+Shift policy, collision checks, text-field/IME protections, disable/reset and per-browser storage. Pick defaults only after checking collisions, or leave the new actions unbound by default.

Relationship to existing work

#214 delivered editable app shortcuts. #642 / #653 propose importing Herdr keymaps; this smaller navigation request should work without a config import or prefix mode.

Evidence: v0.4.6 shortcut registry, pane cycling, and the v0.4.6 palette changes.

Where: Sidebar and navigation; Settings.