Fix image background on multiple screenshots and add text shadow

[russnem]

Summary

• Bug fix: Image backgrounds could only be applied to one screenshot. Switching to another screenshot and uploading or clicking the preview did nothing. Root cause was that the Image object was lost during deep copies (JSON.parse/stringify) and IndexedDB serialization. Fixed by storing the data URL as imageSrc alongside the object and reconstructing it after round-trips. Also reset the file input after upload so the same file can be re-selected.
• New feature: Added a Text Shadow section to the Text tab with controls for color, blur, opacity, and X/Y distance. Applies to both headline and subheadline. Saved per-screenshot. Follows the same collapsible toggle pattern as existing Shadow/Border sections.

Test plan

• Upload an image background on screenshot 1, verify it renders
• Switch to screenshot 2, upload the same image — verify it applies
• Switch back to screenshot 1 — verify the image is still there
• Reload the page — verify image backgrounds persist across sessions
• Enable Text Shadow toggle, adjust color/blur/opacity/distance — verify shadow renders on canvas
• Verify text shadow works on both headline and subheadline
• Verify text shadow settings persist per-screenshot and across page reloads
• Export a screenshot with text shadow — verify it appears in the exported PNG