feat: set window title to reflect selected note

[j4rviscmd]

Summary

• Display "Scripta - {note title}" in the native window title bar, updating dynamically when the user switches notes or after auto-save completes
• Uses Tauri's getCurrentWindow().setTitle() API with proper stale closure guards and error handling
• Falls back to "Scripta - Untitled" when no note is selected

Changes

• src/App.tsx: Add useEffect that watches selectedNoteId and refreshKey to update the window title via Tauri Window API
• src-tauri/capabilities/default.json: Add core:window:allow-set-title permission

Test plan

• Select a note and verify the window title updates to "Scripta - {title}"
• Edit the heading of a note and verify the title updates after auto-save
• Delete all notes and verify the title shows "Scripta - Untitled"
• Create a new note and verify the title shows "Scripta - Welcome to Scripta"

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