Feature/data portability export import

[SiddheshCodes4554]

This PR adds a built-in data portability flow so users can back up, migrate, and restore their memories directly from the web app. The goal is to give users better control over their long-term memory data without needing external scripts or manual database access.

What’s included

• Export memories as JSON.
• Export memories as Markdown.
• Filter exports by container tags.
• Filter exports by date range.
• Import previously exported JSON archives.
• Add a dedicated Data portability section under Settings.

How it works

The web app now exposes authenticated memory export/import routes that proxy through the existing backend API using the same session/auth context as the rest of the app.

For export:

• The route fetches paginated memory documents from the backend.
• JSON exports include metadata and document records.
• Markdown exports render a human-readable archive with headings, timestamps, tags, and content sections.

For import:

• The route accepts previously exported JSON payloads.
• It validates and normalizes the payload before sending it to the backend in batches.
• Imported memories can preserve content, metadata, custom IDs, and tags where available.
• The UI lets users upload a JSON backup and receive feedback on success or failure.

UI changes

A new Settings panel was added for:

• Choosing export format.
• Filtering by date range.
• Filtering by tags.
• Downloading the exported archive.
• Uploading a JSON backup for restore.

Why this change

Many users use supermemory as a long-term memory store for AI workflows. A portability workflow makes it easier to:

• Migrate between environments.
• Keep backups for recovery.
• Archive personal knowledge.
• Increase transparency and user trust.