Add keyboard shortcut for session search

[kvanland]

Related issue

Closes #1059

Summary

• Adds Cmd/Ctrl+Shift+F to open the sidebar if needed, focus the sidebar "Search sessions" input, and select the current query.
• Documents the shortcut in the keyboard shortcuts dialog while leaving Cmd/Ctrl+K available for a future command palette.
• Adds Vitest and Playwright coverage for the shortcut path.

Type of change

• Bug fix
• Feature
• Refactor / chore
• Docs
• Test / CI
• Breaking change

Test coverage

• Unit tests added / updated
• Integration tests added / updated
• E2E tests added / updated
• Manual verification completed
• Existing tests cover this change
• Not applicable

Coverage rationale

Vitest coverage exercises the hotkey predicate, Cmd/Ctrl variants, ignored chords, sidebar open-before-focus behavior, actual search input focus/selection, and the keyboard shortcuts dialog row.

Playwright coverage in tests/e2e_ui/sessions/test_sidebar_search.py verifies Ctrl+Shift+F opens a collapsed sidebar and focuses/selects the search input in the browser.

Validated locally:

• npm test -- src/hooks/useSessionSearchHotkey.test.tsx src/components/KeyboardShortcutsDialog.test.tsx src/shell/Sidebar.test.tsx src/lib/panelSizePreferences.test.ts src/hooks/useResizableSidebar.test.tsx
• npx oxlint src/hooks/useSessionSearchHotkey.ts src/hooks/useSessionSearchHotkey.test.tsx src/shell/Sidebar.tsx src/shell/Sidebar.test.tsx src/components/KeyboardShortcutsDialog.tsx src/components/KeyboardShortcutsDialog.test.tsx src/test-setup.ts
• npm run type-check
• npm run format:check -- src/hooks/useSessionSearchHotkey.ts src/hooks/useSessionSearchHotkey.test.tsx src/shell/Sidebar.tsx src/shell/Sidebar.test.tsx src/components/KeyboardShortcutsDialog.tsx src/components/KeyboardShortcutsDialog.test.tsx src/test-setup.ts
• npm run build
• uv run --python /opt/homebrew/bin/python3.13 pytest tests/e2e_ui/sessions/test_sidebar_search.py --ui-skip-build -q

Note: full npm run lint currently reports existing errors outside this change; focused oxlint on the touched TS/TSX files passes.