fix: surface streamed load failures inline

The streamed loaders on bookmarks, collections, library, search, and the
collection detail page swallowed only ApiError into their in-band `error`
field and re-threw anything else. A raw network failure (a non-ApiError
TypeError from fetch) therefore escaped the `{#await}` to SvelteKit's
generic error page. On the collection detail page a content-load failure
showed a misleading "this collection is empty", and on search a failed
results query showed a false "no matches" because the streamed `error`
was never rendered.

Fold non-ApiError failures into the same in-band `error` so every
transient blip renders as an inline message, add a `contentError` branch
to the collection detail grid, and render the search results `error` in
both result views.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.122.0"
+version = "0.122.1"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.122.0"
+version = "0.122.1"
 edition = "2021"
 default-run = "mangalord"