chore: drop E shortcut for preview, keep only Space

E was a Lightroom holdover and overlapped with the natural rating /
flag culling shortcuts. Space is the only binding now (double-click on
a thumbnail still works). Hints pill updated.

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

frontend/src/components/KeyboardHints.tsx

@@ -13,14 +13,14 @@ export function KeyboardHints() {
         { key: '1-5', action: 'Rate' },
         { key: 'P', action: 'Pick' },
         { key: 'X', action: 'Discard' },
-        { key: 'E / Space', action: 'Preview' },
+        { key: 'Space', action: 'Preview' },
         { key: 'Esc', action: 'Deselect' },
       ]
     : [
         { key: '↑↓←→', action: 'Navigate' },
         { key: 'Click', action: 'Select' },
         { key: 'Shift+Click', action: 'Range' },
-        { key: 'E / Space', action: 'Preview' },
+        { key: 'Space', action: 'Preview' },
         { key: '\\', action: 'Filters' },
         { key: '/', action: 'Search' },
       ]