[Feature] Add Shift+Tab permission mode cycling in Desktop app

Summary

In the CLI and JetBrains, Shift+Tab cycles through permission modes (default → acceptEdits → plan → ...). In the Desktop app, there is no keyboard shortcut — the only way to switch modes is via the dropdown selector next to the send button.

Expected behavior

Shift+Tab (or a configurable shortcut) should cycle permission modes in the Desktop app, matching the CLI behavior.

Current behavior

• CLI/JetBrains: Shift+Tab cycles modes ✅
• VS Code: click on mode indicator (no shortcut, but integrated in the IDE)
• Desktop: dropdown selector only, no keyboard shortcut ❌

Pressing Shift+Tab in Desktop either shows "PR not yet created" (when the text field is focused) or navigates between focusable elements in the chat (standard Electron tab behavior).

Motivation

Switching permission modes is a frequent action during a session (e.g., going from default to acceptEdits when iterating, or switching to plan mode for exploration). A keyboard shortcut would significantly improve the workflow for Desktop users.

The docs explicitly mention "Use the mode selector next to the send button" for Desktop, confirming this is a missing feature rather than a bug.