fix(templ): stop emitting literal { expr } text into attributes and body classes

templ does not interpolate { expr } inside quoted attribute values — the
braces ship as literal text (AGENTS.md gotcha #1). Swept every templ
source for the pattern and fixed all of it:

Body theme classes: 22 page bodies rendered class="theme-{ user.Theme }"
literally, so every page first-painted the :root default (Tokyo Night)
and flashed when main.js applied the real theme. All now render
class={ "theme-" + user.Theme } server-side.

Attribute interpolations (26 across 8 files):
- api_explorer: initAPIExplorerDoc path/body/response and the Copy
  Request/Response buttons (invalid JS, silently dead)
- collections: book-selection :checked/@change/@click ids and four
  hx-get search URLs (collection_id={ collection.ID })
- conflicts: Resolve button id
- devices: Kobo sync-url/auth-token element ids (copy lookups only
  worked because the id and the lookup were the same literal)
- unlinked_books: search-matches/manual-link clicks and container ids
- queue: status/priority badge classes never matched their CSS
- dashboard: Restore-section tooltip text
- collection_rules/collections: collection icon color

Free text goes through the new templates.JSONArg (json.Marshal) —
templ.JSONString in this templ version returns (string, error) and
cannot compose in attribute expressions; uuids via device.ID.String().
Includes regen-only drift from running templ generate (error-path
normalization) on files with no source change.

templates/utils.go

@@ -42,6 +42,19 @@ func themeCheckClass(name, current string) string {
 	return " hidden"
 }
 
+// JSONArg marshals v as a JSON literal for embedding inside JavaScript
+// attribute expressions (x-init, @click, …). Unlike templ.JSONString it is
+// single-valued, so it composes in string concatenation. Strings come out
+// quoted and escaped, so free text (titles, filenames) is safe to splice
+// into Alpine expressions.
+func JSONArg(v any) string {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return `""`
+	}
+	return string(b)
+}
+
 func ContainsString(slice []string, item string) bool {
 	for _, s := range slice {
 		if s == item {