Restructure + abstraction, plugin system, settings UI, universal search mode (#351)

Key changes:   

| Category | Lines | What it is |

|--------------------------|--------|----------------------------------------------------------------------|
| Docs | ~2,100 | plugin-settings.md, release-sources-plugin-guide.md,
provider README |
| Settings UI | ~1,650 | Modal, sidebar, field components (TextField,
SelectField, etc.) |
| ReleaseModal | ~1,200 | Universal mode release picker UI |
| Metadata Providers | ~2,100 | Hardcover + OpenLibrary + base classes |
| Core Infrastructure | ~2,150 | Cache decorator, queue, image cache,
models, config |
| main.py | ~1,570 | Flask routes (replaces old app.py but bigger) |
| Orchestrator | ~590 | Download queue management |
| Config/Settings Registry | ~1,400 | Backend settings system |
| Frontend Hooks | ~750 | useSettings, useSearch, useDownloadTracking,
etc. |
| Other Frontend | ~500 | BookGetButton, ReleaseCell, utils |
| Release Sources base | ~320 | Plugin interfaces |

docker-compose.dev.yml

@@ -1,3 +1,4 @@
+# Local development - builds from source with debug enabled
 services:
   calibre-web-automated-book-downloader-dev:
     extends:
@@ -10,5 +11,7 @@ services:
     environment:
       DEBUG: true
     volumes:
-      - /tmp/cwa-book-downloader:/tmp/cwa-book-downloader
-      - /tmp/cwa-book-downloader-log:/var/log/cwa-book-downloader
+      - ./.local/config:/config
+      - ./.local/ingest:/cwa-book-ingest
+      - ./.local/log:/var/log/cwa-book-downloader
+      - ./.local/tmp:/tmp/cwa-book-downloader