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

docs/reference/settings.mdx

@@ -215,7 +215,7 @@ Advanced settings for log streaming behavior and data retention. Most users can
 
 | Setting | Default | Description |
 |---------|---------|-------------|
-| **Developer Mode** | Off | When on, the [Global Observability](/features/global-observability) view switches from polling to real-time SSE streaming |
+| **Developer Mode** | Off | Enables real-time SSE streaming for [Global Observability](/features/global-observability), and activates debug diagnostics (timing, cache state, file resolution) in the server logs |
 | **Standard Log Polling Rate** | 5s | Polling interval in standard mode. Options: `1s`, `3s`, `5s`, `10s`. Disabled when Developer Mode is on. |
 
 ### Data Retention