feat: keep canvas designs editable via the image library

[bhavjsh]

Fixes #499

⚠️ Depends on #487 (merge that first)
Built directly on the canvas editor, so it can't compile without it.

Changes

• Editor designs are now saved so you can edit them again later (not just as a flat image).
• One save flow: canvas ✓ takes you to the filter screen, and its Save saves the e-paper (filtered) image
• Image Library:
  • Edit reopens a design in the canvas, then the filter screen, and updates the same entry (no duplicate).
  • Change Filter reopens an image in the filter screen to re-pick its filter and transfer.
  • Plain imported images can be edited too, they open in the canvas as a single layer so you can add text/stickers.
• Changing the filter starts from the original image, so filters don't stack on top of each other.
• Reopening a badge brings back the same filter, flips, and display you saved.
• Fixed the delete screen title getting cut off and made the Delete button cleaner.

Recording

edit.mp4

Checklist:

• No hard coding: I have used values from constants.dart or localization files instead of hard-coded values.
• No end of file edits: No modifications done at end of resource files.
• Code reformatting: I have formatted the code using dart format or the IDE formatter.
• Code analysis: My code passes checks run in flutter analyze and tests run in flutter test.

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[github-actions]

Build Status

Build successful. APKs to test: https://github.com/fossasia/magic-epaper-app/actions/runs/28860941212/artifacts/8135962095.

Screenshots

Android Screenshots

iPhone Screenshots

iPad Screenshots

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