feat: photo metadata panel in preview view

Extract the single-photo body of RightSidebar into a reusable PhotoInfoPanel
(rating / color / flag / filename / title / notes / tags / EXIF) and mount
it inside PreviewView as a toggleable right-side overlay so the user can
rate, tag, and read EXIF without leaving the loupe.

- New PhotoInfoPanel: self-contained, owns its own queries and mutations,
  takes a single photoId. darkTheme prop reserved for future use.
- RightSidebar: thinned down — delegates the single-select case to
  PhotoInfoPanel, keeps its own slim bulk-action panel for multi-select.
- PreviewView: I toggles the panel; new top-right Info button mirrors it.
- useKeyboardShortcuts: gate the global I (right-sidebar toggle) to grid
  mode so it doesn't double-fire alongside the preview-scoped handler.

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

frontend/src/hooks/useKeyboardShortcuts.ts

@@ -217,9 +217,11 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
     })
   }
 
-  // Toggle sidebars
+  // Toggle sidebars. The right sidebar `i` shortcut is grid-only — in
+  // preview mode the PreviewView mounts its own `i` handler for the
+  // overlay info panel, and we don't want both to fire.
   useHotkeys('tab', onToggleLeftSidebar, HK_OPTS)
-  useHotkeys('i', onToggleRightSidebar, HK_OPTS)
+  useHotkeys('i', onToggleRightSidebar, { ...HK_OPTS, enabled: !isPreview })
 
   // Search focus (/ or Cmd/Ctrl+F).
   const focusSearch = () => {