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/test/textEntry.spec.js

@@ -0,0 +1,31 @@
+// @vitest-environment happy-dom
+import { describe, it, expect } from 'vitest'
+
+import { arrowNavAllowed, hasText, isTextEntry } from '../src/utils/textEntry.js'
+
+describe('textEntry helpers', () => {
+  it('navigates when the target is not a text field', () => {
+    expect(arrowNavAllowed(document.createElement('div'))).toBe(true)
+    expect(arrowNavAllowed(null)).toBe(true)
+  })
+
+  it('navigates when a focused tag input is empty', () => {
+    const input = document.createElement('input')
+    input.value = ''
+    expect(arrowNavAllowed(input)).toBe(true)
+  })
+
+  it('does NOT navigate when the input has text (caret moves instead)', () => {
+    const input = document.createElement('input')
+    input.value = 'naruto'
+    expect(arrowNavAllowed(input)).toBe(false)
+  })
+
+  it('isTextEntry / hasText recognise inputs and textareas', () => {
+    const ta = document.createElement('textarea')
+    ta.value = 'x'
+    expect(isTextEntry(ta)).toBe(true)
+    expect(hasText(ta)).toBe(true)
+    expect(isTextEntry(document.createElement('div'))).toBe(false)
+  })
+})