fix(editor): handle duplicate wallpaper basenames without a sync loop

[itsmunzir]

Refs #130.

Problem

Two related failures around wallpapers.

1. Collision on staging. The backend copies every background into a single
   flat backgrounds/ directory, so a blueprint that lists the same wallpaper
   twice, or two wallpapers that share a filename (e.g. ~/a/wall.png and
   ~/b/wall.png), is rejected with a basename collision. The editor happily
   accepted such a list and only failed when the user applied the theme.
2. Repeated state echo. A backend-pushed ipc-state-changed snapshot was
   mirrored into the stores unconditionally. Re-writing equal arrays/objects
   re-ran App's SyncState $effect, which pushed the same snapshot straight
   back to Go and produced another echo.

Solution

Deduplicate where the set can still be acted on, and make the backend mirror
idempotent.

• dedupeAdditionalImages(images, primaryWallpaper) drops entries by full
  path and by basename, first entry wins; the primary wallpaper is seeded into
  the seen sets so an additional image cannot shadow it.
• loadBlueprintIntoEditor runs the filter and shows Skipped N duplicate wallpaper(s) instead of the generic load toast when entries were dropped.
• addAdditionalImage returns whether the file was accepted;
  AdditionalImages.svelte toasts Skipped — another wallpaper already uses that filename rather than reporting a successful add.
• setWallpaperPath, restoreHistorySnapshot, setAdditionalImages and
  swapMainWithAdditional reuse the filter, so undo and swap cannot restore a
  colliding pair.
• applyBackendState compares each pushed value against local state
  (sameSerialized for maps, sameStringList for slices) and skips no-op
  writes; App.svelte keeps a lastSyncedSignature and only dispatches
  syncStateToBackend when the snapshot actually changed.

Verification

Observed while preparing this change:

• vitest run in frontend/: 122 tests pass, including the new
  frontend/tests/duplicate-wallpapers.test.ts suite (blueprint load with
  duplicate paths and shared basenames, add/swap/undo, and the backend echo
  round-trip).
• svelte-check: 0 errors, 0 warnings.
• Browser smoke against the Vite dev server: importing a blueprint with a
  duplicated wallpaper keeps exactly one image and shows the skip warning.
• Native Wails backend integration was not run here; the collision itself
  is evidenced by the backend's flat staging directory rather than by a
  reproduced end-to-end run.

Limitations

The reported freeze (#130) is addressed from the state-echo path in the
frontend source; the reporter's exact freeze was not reproduced in a native
Wails build, so this is a source-level fix for the loop plus a concrete fix
for the collision. Reporter confirmation on a native build would be
appreciated.

[itsmunzir]

Gentle check-in — nothing here needs a decision today. The branch is up to date with main and ready whenever you have a look; happy to change anything you want first.