feat(documents): delete original images and show real storage usage

[farfromrefug]

Summary

• New keep original images setting (image settings, on by default). When off, the copy of the source image is no longer stored when a page is created, halving the storage of a kept document.
• Manual delete original images action on selected documents (main list), selected pages and whole document (document view), with confirmation and the freed space reported.
• New documents storage view under settings › storage: total + originals, documents sorted by size, tap to open, selection with delete originals / delete documents, and a "delete all original images" action.
• Lists now show what a document really uses: 1.2 MB (2.4 MB total) — processed images first (what an export weighs), total including originals in parentheses. Backed by a new Page.sourceSize column, backfilled by a migration.
• A page without its original can't be re-cropped or re-transformed: the crop button and the transform rows are hidden, color filters / brightness / contrast stay available. Sync and backup restore handle pages with no original.

Storage location settings moved from the Android-only data section into the new storage section, now visible on both platforms.

Testing

• npx vitest run — 273 pass, including new unit tests for app/utils/originals.ts
• yarn svelte-check clean (same 2 pre-existing errors as main), npx eslint clean on changed files
• ⚠️ Visual check still required — not run on device/emulator. Worth verifying: capture a page with the setting off (page folder should only hold image.jpg), the manual action + freed size snack, the storage view layout/selection, and a sync round-trip of a document whose originals were deleted. The addPageSourceSize migration should also be checked on a large library (it stats every original once).

Refs #706