Fix concurrency, security, and correctness issues across backend and frontend

- C1: Add sync.RWMutex to vm.Manager to protect concurrent vms map access
- H1: Fix IP arithmetic overflow in network slot addressing (byte truncation)
- H5: Fix MultiplexedChannel.Fork() TOCTOU race (move exited check inside lock)
- H8: Remove snapshot overwrite — return template_name_taken conflict instead
- H9: Wrap DeleteAccount DB ops in a transaction, make team deletion fatal
- H10: Sanitize serviceErrToHTTP to stop leaking internal error messages
- H11: Add deleted_at IS NULL to GetUserByEmail/GetUserByID queries
- H12: Add id DESC to audit log composite index for cursor pagination
- H15: Delete dead AuthModal.svelte component
- H17: Move JWT from WebSocket URL query param to first WS message
- H18: Fix $derived to $derived.by in FilesTab breadcrumbs

db/migrations/20260310094104_initial.sql

@@ -171,7 +171,7 @@ CREATE TABLE audit_logs (
     metadata      JSONB NOT NULL DEFAULT '{}',
     created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
-CREATE INDEX idx_audit_logs_team_time ON audit_logs(team_id, created_at DESC);
+CREATE INDEX idx_audit_logs_team_time ON audit_logs(team_id, created_at DESC, id DESC);
 CREATE INDEX idx_audit_logs_team_resource ON audit_logs(team_id, resource_type, created_at DESC);
 
 -- sandbox_metrics_snapshots