fix(security): resolve critical audit findings in marketplace and frontend

CRITICAL fixes:
- marketplace.rs: Replace fake SHA-256 (SipHash) with real sha2 crate
- marketplace.rs: Reject empty checksums (refuse unsigned plugins)
- marketplace.rs: Add install path traversal protection (reject ../|/|\)
- marketplace.rs: Add HTTPS-only URL validation on download_url
- marketplace.rs: Add curl --proto =https to block file:/gopher: SSRF
- marketplace.rs: Add --max-filesize 50MB download cap
- marketplace.rs: Add --no-same-owner --no-same-permissions to tar extraction
- marketplace.rs: Post-extraction path validation (canonicalize check)

Frontend fixes:
- pro-bridge.ts: Rename Symbol→CodeSymbol (avoid global collision)
- pro-bridge.ts: Tighten trust type to union 'human'|'agent'|'auto'
- PluginMarketplace.svelte: URL sanitization (reject non-https hrefs)

Remaining audit fixes (HIGH/MEDIUM/LOW) being applied by background
agents — will be committed separately when complete.

Cargo.lock

@@ -67,6 +67,7 @@ dependencies = [
  "rusqlite",
  "serde",
  "serde_json",
+ "sha2",
  "tauri",
  "tokio",
 ]