Fix(gallery): Re-add image browsing with arrow keys

[DustyShoe]

Summary

This PR restores and hardens keyboard image browsing in Gallery/Viewer flows, including when panels are collapsed and when invoking from prompt-focused inputs.

Key Changes

1. Restored arrow-key browsing behavior

• Gallery arrow navigation works again in viewer-capable contexts.
• No longer requires clicking a gallery item first in common viewer flows.

2. Viewer-side Left/Right navigation when gallery is unavailable

• Left/Right now navigate prev/next image when:
  • fullscreen panel mode is active, or
  • right panel is collapsed, or
  • gallery panel is collapsed.
• Prevented handler conflicts by making grid hotkeys yield in viewer-arrow-navigation mode.

3. Paged Gallery cross-page keyboard navigation

• In paged mode, keyboard navigation indexes against full image list.
• Crossing page boundaries via arrows automatically changes page/selection correctly.

4. Shared prev/next navigation logic

• Extracted shared hook:
  • useNextPrevItemNavigation
• Both UI arrows (NextPrevItemButtons) and viewer hotkeys call the same prev/next logic.

5. Navigation API additions

• Added helpers in navigation-api:
  • isFullscreen(tab)
  • isRightPanelCollapsed(tab)
  • isGalleryPanelCollapsed(tab)
  • isViewerArrowNavigationMode(tab)

6. Invoke focus behavior (Generate + Upscaling)

• After invoke (button or hotkey), prompt/input focus is blurred and focus region is set to viewer in:
  • Generate
  • Upscaling
• Ensures arrow browsing is immediately available after invoke from text input.

Related Issues / Discussions

Fixes #8589

QA Instructions

A. Baseline Gallery Keyboard Nav

• Select image in visible gallery.
• Press Left/Right/Up/Down.
• Confirm selection moves as expected.

B. Viewer Without Extra Click

• Open viewer-capable tab.
• Do not click gallery item.
• Press arrows.
• Confirm browsing still works.

C. Right Panel Collapsed Only

• Collapse only right panel (G).
• Press Left/Right.
• Confirm prev/next image navigation works.

D. Fullscreen / Panels Hidden

• Toggle fullscreen panels mode (F).
• Press Left/Right.
• Confirm behavior matches viewer prev/next UI controls.

E. Paged Gallery Boundary

• Enable paged gallery view.
• Navigate to page boundary and press Left/Right.
• Confirm automatic jump to previous/next page selection.

F. Invoke From Prompt Input

• Focus prompt textarea in Generate, press Ctrl+Enter (Cmd+Enter on mac).
• Confirm input focus is cleared and arrow browsing works immediately.
• Repeat for Upscaling.
• Repeat with Ctrl+Shift+Enter / Cmd+Shift+Enter.

G. Input Safety

• Focus an input/textarea in contexts where viewer-arrow mode is not active.
• Confirm arrows behave as text-cursor keys (no unintended image nav).

Merge Plan

Should be simple merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)