feat: add timeline sidebar navigation with note grouping

[j4rviscmd]

Summary

• Add a collapsible sidebar using shadcn/ui Radix Sidebar that displays notes grouped by relative date (Today, Yesterday, Previous 7 Days, Older)
• Notes can be selected from the sidebar to load in the editor
• • button immediately creates a new "Untitled" note and selects it
• Last opened note ID is persisted in localStorage and restored on startup
• Consolidate duplicate DEFAULT_CONTENT into a shared editor/lib/constants.ts module
• Fix stale closure bug in useAutoSave by adding onNoteSaved to dependency array

Changes

Area	Change
Sidebar	New NoteSidebar component with date-grouped note list
Editor	Extract shared DEFAULT_BLOCKS / DEFAULT_CONTENT constants
Auto-save	Fix onNoteSaved stale closure; simplify unmount cleanup
Layout	Wire SidebarProvider + SidebarInset + SidebarTrigger in App
shadcn/ui	Add sidebar, sonner, button, input, separator, skeleton, tooltip, sheet components

Test plan

• Verify notes appear grouped by date on sidebar load
• Click a note to load it in the editor
• Press + button to create a new note immediately (no typing required)
• Verify "Untitled" new note appears in sidebar and is selected
• Verify title updates in sidebar after editing heading and saving
• Close and reopen app — last opened note should be restored
• Verify sidebar toggle via hamburger button

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