feat: add theme toggle and thumbnail labels

[CodeWithDennis]

Summary

• Light / dark / auto UI theme in the header, with persistent preference and system theme when Auto
• Overlay labels on thumbnails: Left/Right (or Top/Bottom), optional badge, per-label position, size slider
• Labels are included in export and saved in presets; dark tokens and button styling work in dark mode

Key changes

• resources/views/app.blade.php: theme segment, Labels panel, boot-screen dark styles, FOUC prevention
• resources/js/app.js: theme state + localStorage, label settings in preview/export/presets
• resources/js/compositor.js: draw pill labels after the split/overlap render
• resources/css/app.css: missing dark tokens, color-scheme, label input styles

Behavior / impact

• Default theme is Auto (follows OS); choice is stored in pairframe.theme
• Labels are off by default; when On they appear in preview and export
• Per-label position: Top/Middle/Bottom (for horizontal: Left/Center/Right)

Test plan

• Switch theme Light / Dark / Auto; reload keeps the choice
• Auto follows OS dark/light (and updates live on OS change)
• Labels On: Left/Right/Badge text visible on preview
• Diagonal + different positions per label
• Horizontal: Top/Bottom labels + Left/Center/Right positions
• Export PNG/JPG includes the same labels
• Save/load preset keeps label settings
• Dark mode: buttons, inputs, and panels stay readable