fix(stacks): harden stack management with security, validation, and logging (#520)

* fix(stacks): harden stack management with security fixes, validation alignment, and logging

Validate WebSocket stack names with isValidStackName() to close a
path-traversal gap on the /api/stacks/:stackName/logs WS endpoint.
Align POST /api/stacks to use the canonical validator (allows underscores).
Replace error: any catch blocks with error: unknown + type narrowing.
Add cache invalidation to PUT /api/stacks/:stackName/env.
Rename DELETE param from :name to :stackName for consistency.

Add standard [Stacks] lifecycle logs and diagnostic [Stacks:debug] logs
gated behind the Developer Mode toggle (with 5s TTL cache).
Extract shared isDebugEnabled() and getErrorMessage() utilities.

Frontend: roll back optimistic status on API failure, guard unsaved
changes when switching stacks, pre-check duplicate names in App Store.

* docs(settings): update Developer Mode description to mention debug diagnostics

frontend/src/components/AppStoreView.tsx

@@ -138,6 +138,19 @@ export function AppStoreView({ onDeploySuccess }: AppStoreViewProps) {
             toast.error("Stack name is required");
             return;
         }
+
+        // Pre-check for duplicate stack name
+        try {
+            const checkRes = await apiFetch('/stacks');
+            if (checkRes.ok) {
+                const existingStacks: string[] = await checkRes.json();
+                if (existingStacks.includes(stackName.trim())) {
+                    toast.error(`A stack named "${stackName.trim()}" already exists. Choose a different name.`);
+                    return;
+                }
+            }
+        } catch { /* proceed to deploy; backend will catch duplicates */ }
+
         setIsDeploying(true);
 
         const modifiedTemplate = { ...selectedTemplate };