Add local Markdown notes linked to pages

Summary

Add local Markdown notes that can be linked to the current page.

Scope

• Store notes as user-readable Markdown files with a rebuildable index.
• Provide a native Photon notes page or compact utility for finding, creating, and editing notes.
• Let a note link to the current page and open that page again.
• Detect edits made outside Photon and avoid silently overwriting conflicting changes.
• Keep note storage separate from Engine page data and do not require an account or remote service.

Acceptance criteria

• Notes remain readable and editable outside Photon.
• External file changes appear in Photon without restarting.
• Conflicting edits are surfaced before saving.
• Private windows do not create persistent notes from private browsing context.

Design reference: Zephium uses Markdown files as the source of truth and rebuilds note metadata from them: https://github.com/zephium-browser/Zephium/blob/main/crates/zephium-core/src/notes.rs