fix(modal): next arrow clears metadata panel + arrows work in empty tag input

Image viewer (#609):
- The next (▶) arrow was offset from the viewport edge (right:16px) so it
  floated over the 320px metadata side panel. Offset it off a shared
  --fc-side-w var so it sits at the image's right edge instead; full-width
  again below 900px when the panel stacks under the image.
- Arrow nav was fully disabled whenever a text field was focused. Now it
  yields to the caret ONLY when the field has text; an empty tag-entry field
  still navigates ←/→. Extracted to utils/textEntry.js (arrowNavAllowed).
  ESC behaviour unchanged (already closes the modal, overlay-aware).

Test: arrowNavAllowed — empty/non-text → navigate, text present → don't.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/utils/textEntry.js

@@ -0,0 +1,21 @@
+// Helpers for deciding whether a keyboard shortcut should fire or yield to a
+// focused text field.
+
+export function isTextEntry (el) {
+  if (!el) return false
+  const tag = el.tagName
+  return tag === 'INPUT' || tag === 'TEXTAREA' || el.isContentEditable
+}
+
+export function hasText (el) {
+  if (!el) return false
+  if (el.isContentEditable) return (el.textContent || '').length > 0
+  return (el.value || '').length > 0
+}
+
+// Prev/next arrow navigation should fire UNLESS focus is in a text entry that
+// already has content — in that case the arrows belong to the caret so it can
+// move through the text. An empty (or non-text) target still navigates.
+export function arrowNavAllowed (target) {
+  return !(isTextEntry(target) && hasText(target))
+}