Add photo upload UI to community fridge form

[neal-bpm]

Describe your changes

Replaces the plain "Photo URL" text input in the community fridge form with a proper drag-and-drop file upload widget. The widget is built on Phoenix LiveView's native file upload primitives and persists files via MediaStorage. It handles new uploads, preserves existing photos through re-submits, supports cancellation of pending entries before save, and validates file type and size client-side.

• mount/1 registers a single-file upload slot (:photo, gif/png/jpg/jpeg and size of 10MB)
• Save handler calls consume_uploaded_entries → MediaStorage.upload_file! and injects the returned URL into params; falls through unchanged when no file is selected so the hidden input preserves the existing photo
• cancel_upload event removes a pending entry before save
• Template shows existing photo preview (with hidden input to preserve it through form submits), drag-and-drop zone with live_file_input, pending entry preview via live_img_preview, and per-entry error display

Tests cover: existing photo renders in edit form, dropzone markup is present, pending upload preview appears after file selection, and cancel removes the preview.

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added tests.
• Are there other PRs or Issues that I should link to here?
• Will this be part of a product update? If yes, please write one phrase
  about this update in the description above.

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[neal-bpm]

Superseded by #540. Work consolidated into vn/community-fridge-feature for unified review.