refactor: split App.tsx desktop-shell orchestration into smaller hooks/components

[sauravpanda]

Summary

src/App.tsx has become the orchestration point for too many unrelated desktop-shell concerns: lazy imports, panel state, command registration, keyboard shortcuts, worktree switching, sidebar actions, and top-level content selection all live in one file.

Evidence

• src/App.tsx is currently over 2,000 lines long.
• The file declares a very large set of lazy-loaded panels and also coordinates their open/close behavior.
• Command palette registration, global shortcut wiring, dashboard actions, content-tab routing, and sidebar action exposure are all handled in the same component.
• Recent bug work in this area already overlaps shortcut closures and render-time side effects, which suggests the shell controller is hard to reason about and hard to change safely.

Impact

• The main app shell is expensive to understand and risky to modify.
• Small feature additions tend to expand the central controller instead of staying local to a feature boundary.
• Bugs in shell behavior are more likely to cross-cut unrelated panels and workflows.

Suggested fix

• Extract shell concerns into focused hooks/components such as command registration, shortcut handling, panel orchestration, and project/worktree selection.
• Keep App.tsx as a composition layer rather than the behavioral home for every desktop feature.
• Add a lightweight architecture note describing the intended ownership boundaries for shell state.

Acceptance criteria

• App.tsx is materially smaller and mostly declarative.
• Command/shortcut/panel behavior is covered by clearer ownership boundaries.
• New shell features can be added without expanding one central controller file.