Shortcuts: add next/previous tab and next/previous workspace bindings #2038

src/lib/shortcuts.ts (SHORTCUTS) currently provides previous-pane (Mod+Shift+ArrowUp) and next-pane (Mod+Shift+ArrowDown) to cycle through panes in the active list, plus new-session and new-tab, but has no keyboard shortcuts to switch between tabs in the active workspace or between Herdr workspaces.

Adding bindable shortcuts in src/lib/shortcuts.ts and Settings -> Shortcuts for:

• previous-tab / next-tab (cycle tabs in the current workspace, matching TabStrip order)
• previous-workspace / next-workspace (cycle workspaces in Sidebar order)

would let keyboard users move across tabs and Herdr workspaces without opening the Command Palette or reaching for the mouse.