feat(activity): add CSV and JSON exports

[Hallab7]

Activity export (CSV / JSON)

Summary

Adds client-side CSV and JSON exports for Stellar activity.

Exports use the exact wallet, type, and status-filtered entries currently shown
on the Activity page. This gives users a spreadsheet-friendly format for
accounting and a schema-backed JSON format for programmatic replay.

What changed

• Added a new /activity page and navigation entry
• Added CSV and JSON export controls
• Export buttons operate on the current filtered view only
• Added RFC 4180-compatible CSV serialization
• Added a UTF-8 BOM to CSV downloads for reliable non-ASCII text handling in
  desktop Excel
• Added CSV formula-injection protection for memo values beginning with:
  • =
  • +
  • -
  • @
• Added src/schema/activity.json using JSON Schema draft 2020-12
• Added strict schema-validation and CSV-safety unit tests

CSV format

The CSV export contains these columns:

timestamp,id,chain,wallet,kind,direction,status,amount,token,recipient,memo

Compatibility details:

• Uses CRLF line endings
• Quotes fields containing commas, quotes, or line breaks
• Escapes embedded quotes according to RFC 4180
• Downloads as UTF-8 with a BOM
• Defaults Stellar token labels to XLM when an activity entry does not include
  an explicit token
• Prefixes dangerous memo values with an apostrophe so spreadsheet applications
  treat them as text rather than formulas

JSON format

JSON exports preserve the filtered ActivityEntry[] data without transforming
timestamps or metadata, making the output suitable for replay and programmatic
processing.

The shipped schema validates:

• Required activity identifiers and timestamps
• Supported activity kinds
• Direction and status enums
• Optional amount, token, recipient, and metadata fields
• Rejection of undeclared top-level activity properties

User flow

1. Connect a Stellar wallet
2. Open Activity
3. Select optional type and status filters
4. Choose Export CSV or Export JSON
5. Receive a dated file containing only the displayed filtered entries

Export controls are disabled when the filtered view is empty.

Verification

• CSV-injection coverage for =, +, -, and @ memo prefixes
• CSV commas, quotes, and line breaks are escaped correctly
• JSON output validates against src/schema/activity.json with strict Ajv
• Exporter tests pass: 6/6
• TypeScript check passes
• Production Vite build passes
• Prettier and git diff --check pass
• /activity and its compiled module are served successfully by the Vite
  dev server

Scope

The feature is entirely client-side. It adds no backend, API keys, network
requests, or server-side storage.

closes #100

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[truthixify]

Conflicts against develop from the recent batch. Rebase all three of your open demo PRs (140/141/142) together, force-push each.