feat: show source directories in library stats and settings UI

Add active source root directories to the library stats endpoint and
display them in the settings page. Hardcode container PHOTO_DIRS to
/photos since the volume mount handles host path mapping. Add .env to
.gitignore to prevent committing secrets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/dialogs/SettingsDialog.tsx

@@ -258,6 +258,17 @@ export function SettingsPage() {
                 />
               </div>
 
+              {(libStats?.source_dirs?.length ?? 0) > 0 && (
+                <div className="mt-3 text-xs text-text-muted">
+                  <span className="font-medium text-text">Source folders</span>
+                  <div className="mt-1 space-y-0.5">
+                    {libStats!.source_dirs.map((dir: string) => (
+                      <div key={dir} className="truncate font-mono" title={dir}>{dir}</div>
+                    ))}
+                  </div>
+                </div>
+              )}
+
               {/* Inline scan progress. Rendered as a full-width sub-card
                   when a scan is active so the user sees the same info
                   they'd get from the floating widget without leaving

frontend/src/services/api.ts

@@ -625,6 +625,7 @@ export interface LibraryStats {
   total_videos: number
   total_size: number
   total_size_gb: number
+  source_dirs: string[]
 }
 
 // Heaps API