refactor: remove dead code across server, templ pages, and client modules

Deadcode-verified removals (RTA reachability from main, cross-checked
with -test and textual xrefs):

Go server:
- whole files: middleware/transaction.go, middleware/error_handler.go,
  middleware/password_validator.go (live registration machinery kept),
  handlers/sync.go, sync/offline.go
- unrouted endpoints: auth UpdateUsername/UpdateEmail, timezone
  settings handler, OPDS RegisterOPDS, Kobo LibrarySync handler,
  ReaderHandler.ShowReader, JSON-data twins on collections/library
- duplicate book-matching handlers (book_matching.go Handler twins,
  matching.go MatchingHandler twins), superseded helpers
  (router.parseUUID, handlers.ValidateDeviceToken, normalizePath,
  userExists, generateLockID)

Templ/client:
- never-routed pages: api_explorer.templ, queue.templ and their
  client modules api-explorer.ts, api-explorer-docs.ts, queue.ts
- fully orphaned wrapper modules events.ts, dom.ts
- broken clearSyncQueue (called a nonexistent endpoint) and the
  hidden, never-populated sync-queue section on the devices page
- 166+ unused exports demoted to private (knip-verified); ~85 truly
  dead declarations removed via tsc noUnusedLocals fixpoint
- stale 'interactive API Explorer' claims dropped from docs

The docs drift-gate package (internal/docs), templ-bound modules
(dashboard, stats, header, index, login, profile*, bookshelf,
collections, toast-error), sqlc-generated code, and web/static are
intentionally untouched.

internal/services/book_matching_test.go

@@ -343,24 +343,3 @@ func TestLinkBookRequest_Validation(t *testing.T) {
 		})
 	}
 }
-
-// Example usage
-func ExampleBookMatchingService_QueryBooks() {
-	service := NewBookMatchingService(nil) // Would need real DB
-
-	req := &BookQueryRequest{
-		Identifiers: []string{
-			"uuid:550e8400-e29b-41d4-a716-446655440000",
-			"isbn:9783161484100",
-		},
-		SHA256:   "d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592",
-		Title:    "Test Book",
-		Author:   "Test Author",
-		FileSize: 1024000,
-	}
-
-	_ = service
-	_ = req
-	// response, err := service.QueryBooks(context.Background(), req)
-	// fmt.Printf("Matches: %d, Action: %s\n", len(response.Matches), response.Action)
-}