feat: add settings dialog with theme picker in sidebar footer

[j4rviscmd]

Summary

• Add a new features/settings module with a modal dialog accessible from a settings button at the bottom of the sidebar (SidebarFooter)
• The dialog provides Light/Dark/System theme selection using radio-style ThemeOption components backed by the existing ThemeProvider
• Install shadcn/ui Dialog component (@base-ui/react/dialog primitive)
• Simplify NoteSidebar by extracting a renderNoteItem helper and inlining the unpinnedNotes variable

Changes

File	Change
src/components/ui/dialog.tsx	New — shadcn/ui Dialog component
src/features/settings/index.ts	New — barrel export
src/features/settings/ui/SettingsDialog.tsx	New — settings dialog with theme picker
src/features/settings/ui/ThemeOption.tsx	New — radio-style theme option component
src/features/sidebar/ui/NoteSidebar.tsx	Modified — SidebarFooter + SettingsDialog integration + simplification

Test plan

• Verify settings button (gear icon) appears at the bottom of the sidebar
• Click the button and confirm the dialog opens
• Select each theme option (Light/Dark/System) and verify theme changes immediately
• Close and reopen the dialog to confirm selected theme persists
• Verify sidebar note list, search, pin/delete still work correctly
• Test on both desktop and mobile sidebar layouts

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