Storyboard UX polish: navigation guard, toolbar flip, SelectValue fix, re-render feedback, sidebar redesign

[amoghbanta]

Summary

• Navigation guard: Prompts user before navigating away mid AI-image generation (ref-based guard + window.confirm)
• Toolbar flip: Text element toolbar flips above the element when near the viewport bottom instead of clipping off-screen
• SelectValue fix: Dropdown triggers no longer show the full description text alongside the key (pass children to Radix SelectValue to override item mirroring)
• Re-rendering overlay: After saving sectioning changes, a full-screen overlay with animated dots (same style as AI editing loader) blocks interaction while the page re-renders
• Sidebar redesign: Icon rail + portrait page thumbnails layout; stable 220px width regardless of pages panel state; PanelLeftOpen toggle button; export button moved to pages panel footer to prevent dropdown clipping
• Cleanup: escapeRegex helper (replacing 4× inline copies), unmount abort controller cleanup, removed dead code

Test plan

• Open storyboard → click image → AI → submit prompt → press arrow key while generating → confirm dialog appears; Cancel stays; OK navigates
• Click a text element near the bottom of the preview → toolbar appears above the element (not clipped)
• Open Section Data panel → section type dropdown trigger shows only "chapter" not "chapter Chapter heading"
• Toggle prune on any element → click Save → save bar disappears → re-rendering overlay appears → disappears when done
• Pages panel opens/closes via PanelLeftOpen button; export dropdown opens above the button without being clipped
• pnpm typecheck — clean