feat(metadata): multi-provider abstraction (LoC, DNB, K10plus, Open Library) (#128)

Make Hardcover one metadata source among several. With no Hardcover key
configured, ISBN lookup still works via the keyless library providers.

Backend (src-tauri/src/metadata/): a unified BookMetadata model and three
provider-agnostic Tauri commands behind enum dispatch. A config-driven SRU
endpoint registry powers Library of Congress, Deutsche Nationalbibliothek and
K10plus (MARC21/slim via a quick-xml parser: author inversion, 6xx subjects,
008/041 language, entity decoding, ISBN-match verification). Open Library is a
keyless JSON provider; Hardcover (GraphQL) moves in unchanged in behaviour.
ISBN 10<->13 equivalence lives here as the single authority.

Frontend: a capability-driven provider registry + a streaming aggregator that
dedupes by ISBN to one whole record (with cover backfill as the only borrowed
field) and pins the file's ISBN match. One unified MetadataSearchModal with
source provenance and a designed cover-less placeholder; provider-agnostic
hooks for the edit and import flows; a "Metadata sources" settings pane; and
library subjects offered as opt-in tag-suggestion chips. Language is parsed but
applied later (CDL-2).

Settings migrate from the flat hardcoverApiKey/hardcoverAutoLookup keys to a
provider block (v0->v1, then v1->v2 adds K10plus), with the keyless libraries
enabled by default and existing Hardcover keys preserved.

Also fixes:
- Edit Book showing a stale cover after a lookup: cache-bust the single-book
  cover URL by mtime (the list paths stay stat-free at scale).
- Settings panel content shifting sideways when a scrollbar appears.
- Provider-row switch alignment; Add Book modal label truncation.
- "Matches ISBN" picker badge contrast in light and dark.

Fixes CDL-7

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: phildenhoff

Cargo.lock

@@ -21,6 +21,7 @@ epub = "2.1.1"
 mobi = "0.8.0"
 libcalibre = { path = "../crates/libcalibre" }
 log = "0.4"
+quick-xml = "0.38"
 regex = "1.10.2"
 reqwest = { version = "0.12", features = ["json"] }
 serde = { version = "1.0", features = ["derive"] }