feat: rate limiting + E2E crypto lifecycle test

Rate limiting (closes the last security gap #3):
- New RateLimiter: in-memory fixed-window IP-based limiter (std::sync::Mutex
  HashMap, no new deps). Configurable via RATE_LIMIT_MAX (default 100) +
  RATE_LIMIT_WINDOW_SECS (default 60) env vars.
- general_rate_limit_middleware now reads ClientIp from request extensions and
  rejects with 429 + Retry-After header when over the limit. Wired via
  from_fn_with_state in app.rs. Lived on AppState as Arc<RateLimiter>.
- Deleted the dead auth_rate_limit_middleware (never wired).
- 3 unit tests (allows up to N, independent IPs, window reset).

E2E crypto lifecycle test:
- Full zero-knowledge round-trip against jsdom's real Web Crypto: setup →
  encrypt → verify ciphertext → unlock with password → decrypt → recover via
  phrase → rewrap under new password → decrypt. Plus wrong-password/wrong-phrase
  failures and cross-user key isolation.
- Fixed wrapDek/unwrapDek: base64-encode raw DEK bytes (was using TextDecoder
  which produced non-base64), and make unwrapped DEK extractable (needed for
  rewrapDek to export).

Verified: backend 24 tests 0 warnings; frontend 24 tests, build clean.

backend/src/app.rs

@@ -92,7 +92,7 @@ pub fn build_app(state: AppState) -> Router {
 
     public_routes
         .merge(protected_routes)
-        .with_state(state)
+        .with_state(state.clone())
         .layer(
             ServiceBuilder::new()
                 // Resolve the client IP once and stash it for handlers/middleware
@@ -104,8 +104,9 @@ pub fn build_app(state: AppState) -> Router {
                 .layer(axum::middleware::from_fn(
                     middleware::security_headers_middleware,
                 ))
-                // General rate limiting (currently a stub)
-                .layer(axum::middleware::from_fn(
+                // IP-based rate limiting (reads ClientIp from extensions).
+                .layer(axum::middleware::from_fn_with_state(
+                    state.clone(),
                     middleware::general_rate_limit_middleware,
                 ))
                 .layer(TraceLayer::new_for_http())