fix: checkpoint WebKit LocalStorage WAL on startup to prevent unbounded growth

[kckylechen1]

Problem

WebKit WKWebView uses WAL mode for LocalStorage SQLite databases but never checkpoints them. When the app writes large blobs frequently (e.g. Windsurf/Codex account caches with quota snapshots, written every few minutes via auto-refresh), the WAL file grows unbounded.

Observed on my machine: 13 GB WAL file for only ~5 MB of actual data (45 key-value pairs).

~/Library/WebKit/com.jlcodes.cockpit-tools/WebsiteData/.../LocalStorage/
  localstorage.sqlite3     5.0 MB   (actual data)
  localstorage.sqlite3-wal  13 GB   (should be ~0)
  localstorage.sqlite3-shm  27 MB

After running PRAGMA wal_checkpoint(TRUNCATE) manually, the WAL dropped to 0 bytes.

Root Cause

The frontend writes full JSON-serialized account arrays to localStorage on:

• Every page navigation/mount
• Every auto-refresh cycle (configurable, ~5-30 min per platform)
• Every user action (switch account, refresh token, etc.)

11 platform account stores + 11 instance stores = 22 caches being rewritten continuously. The heaviest key (agtools.windsurf.accounts.cache at ~5 MB) includes raw API response blobs (copilot_quota_snapshots, windsurf_user_status, etc.) with no size reduction.

WKWebView doesn't auto-checkpoint its SQLite databases, so the WAL accumulates indefinitely.

Fix

Add a startup maintenance step (modules/webkit_cache_maintenance.rs) that:

1. Locates all localstorage.sqlite3 files under the WebKit data directory
2. Runs PRAGMA wal_checkpoint(TRUNCATE) on each
3. Runs in a background thread to avoid blocking startup
4. Logs success/failure for observability

Runs at startup in lib.rs setup, before the WebView is fully loaded — so the lock is available.

Testing

• Compiled successfully with cargo check (no new warnings)
• Manual test: WAL went from 13 GB → 0 bytes after checkpoint

Future Improvements (not in this PR)

• Add periodic checkpoint during runtime (e.g. every 30 min)
• Strip large API response blobs before persisting to localStorage
• Add size-based eviction for old cache entries
• Consider moving heavy caches to Tauri's Rust-side SQLite instead of WebKit LocalStorage

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