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  matching.go MatchingHandler twins), superseded helpers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/sync/kepub_cfi_converter.go

@@ -1,7 +1,6 @@
 package sync
 
 import (
-	"fmt"
 	"log"
 
 	"golang.org/x/net/html"
@@ -13,10 +12,10 @@ type KEPUBCFIConverter struct {
 }
 
 type KEPUBConversionResult struct {
-	CFI               string
-	ExtractedContext  string
-	Percentage        float64
-	Precision         string
+	CFI              string
+	ExtractedContext string
+	Percentage       float64
+	Precision        string
 }
 
 func NewKEPUBCFIConverter(epubPath, kepubPath string) *KEPUBCFIConverter {
@@ -265,57 +264,6 @@ func extractSurroundingText(textNode *html.Node, offset int, window int) string
 	return normalizeWhitespace(string(allRunes[start:end]))
 }
 
-func (k *KEPUBCFIConverter) ComputeKEPUBPercentage(kepubCFI string) (float64, error) {
-	if !IsStandardEPUBCFI(kepubCFI) {
-		return -1, fmt.Errorf("not a standard epubcfi")
-	}
-
-	spineIndex, steps, err := parseEPUBCFI(kepubCFI)
-	if err != nil {
-		return -1, err
-	}
-
-	kepubDoc, _, docErr := k.kepubConverter.getContentDoc(spineIndex + 1)
-	if docErr != nil {
-		return -1, docErr
-	}
-
-	kepubBody := findBody(kepubDoc)
-	if kepubBody == nil {
-		return -1, fmt.Errorf("no body in kepub doc")
-	}
-
-	textNode, textOffset, resolveErr := resolveCFIToNode(kepubBody, steps)
-	if resolveErr != nil {
-		return -1, resolveErr
-	}
-
-	charOffset := countTextCharsBefore(textNode) + textOffset
-
-	total := 0
-	kepubSpine, spineErr := k.kepubConverter.loadSpine()
-	if spineErr != nil {
-		return -1, spineErr
-	}
-
-	for i := range kepubSpine.items {
-		doc, _, dErr := k.kepubConverter.getContentDoc(i + 1)
-		if dErr != nil {
-			continue
-		}
-		b := findBody(doc)
-		if b != nil {
-			total += countTextChars(b)
-		}
-	}
-
-	if total <= 0 {
-		return -1, fmt.Errorf("no text in kepub")
-	}
-
-	return float64(charOffset) / float64(total), nil
-}
-
 func init() {
 	_ = log.Printf
 }