feat: add Sub2API multi-site, multi-key usage tracking

[JaminYe]

Summary

Pane users with Sub2API keys currently need to open each site's dashboard to inspect their usage. This adds a dedicated Sub2API provider with multiple sites and keys, with one stable card per key.

• Query only the configured site's GET /v1/usage with that key's Bearer credential. Display total and rolling key quotas, daily/weekly/monthly subscription allowances, wallet balances (including debt), expiry, and collapsible request/token/actual-cost summaries.
• Reuse the site/key settings workflow, preserve card identity across edits, clear old usage on credential or origin changes, and discard late results from obsolete contexts. Failed refreshes retain explicitly stale history.
• Project the tightest valid allowance into the tray, reuse quota alerts, and expose sanitized per-key local HTTP usage. Shared wallets/subscriptions are not summed into Total Spend.
• Store credentials in owner-only local files, disable redirects, bound requests and response sizes, and exclude Sub2API from telemetry. Document the manual-key exception precisely in the contribution rules.

Validation

• cargo test --manifest-path src-tauri/Cargo.toml sub2api --lib: 28 passed. Windows credential ACL tests passed when rerun outside the sandbox.
• node scripts/sub2api-display.test.mjs: 8 passed.
• npx tsc --noEmit: passed.
• git diff --cached --check: passed before commit.

Tests use local mock endpoints and temporary credentials; no live paid services were queried. Desktop layout, tray appearance, and settings interactions still need manual visual validation; screenshots are not included.

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