App Theme UI Improvements (litmus-r6e1)

Restructure ThemeCard, add ShortlistCheckbox, rework sidebar, rename button text, fix tooltip clipping

Summary of Changes

1. ThemeCard restructured (theme_list.rs)

• Link now wraps only the card body (header + preview + swatches), not the footer actions
• Action buttons sit outside the Link in a .theme-card-actions div with app theme colors

2. ShortlistCheckbox added (components.rs)

• New ShortlistCheckbox component with <label><input type=checkbox> Shortlist</label>
• Used on browse page cards; ShortlistToggle (button) kept for detail page
• Renamed “Use as App Theme” → “Apply”, “✓ App Theme” → “✓ Applied”

3. Sidebar reworked (sidebar.rs)

• Removed compare checkboxes (checked: HashSet) and all checkbox-based gating
• App theme pinned at top of shortlist with “current” badge
• Compare URL = app theme + shortlist slugs (deduped); shown when 2+ items

4. Tooltip clipping fixed (scene_renderer.rs, style.css)

• line_idx forwarded to SpanView; lines 0-1 get contrast-tooltip-below class
• .scene-block pre: overflow-x: clip; overflow-y: visible instead of overflow-x: auto
• .contrast-issues-list: increased max-height, overflow-y: visible

5. CSS updates (style.css)

• .theme-card-actions gets explicit app theme bg/fg/border
• .shortlist-checkbox styles
• .sidebar-current-badge and .sidebar-shortlist-name-link styles
• Removed old .sidebar-shortlist-check styles