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/settings/DeveloperSection.tsx

@@ -66,7 +66,7 @@ export function DeveloperSection({ settings, onSettingChange, onSave, isSaving,
                         <div className="flex items-center justify-between">
                             <div className="space-y-0.5">
                                 <Label htmlFor="developer_mode" className="text-base">Developer Mode</Label>
-                                <p className="text-xs text-muted-foreground">Enable Real-Time Metrics & Extended Logs</p>
+                                <p className="text-xs text-muted-foreground">Enable Real-Time Metrics, Debug Diagnostics & Extended Logs</p>
                             </div>
                             <Switch
                                 id="developer_mode"