feat(admin): editable tunable settings UI with grouped sub-sections

Replace the read-only "System Information" card (which listed hardcoded
values) with editable HTMX forms, organized so the live vs restart
distinction and related settings are visually clear.

admin_settings.templ:
- AdminSettings signature now takes liveGroups and restartGroups
  ([]SettingGroup) instead of a flat entry list.
- Remove the static System Information list. Render two cards: "Live"
  (green, applies immediately) and "Restart Required" (warning header,
  saved but only takes effect after restart).
- Within each card, TunableSettingsSection clusters entries into
  labeled sub-sections by Group (e.g. "Password Quality", "Device Rate
  Limits", "Login Lockout", "Worker Pool") with uppercase tracked
  sub-headers.
- TunableSettingRow renders an inline HTMX form per setting: a Yes/No
  select for bools, a number input with min/max for ints, text
  otherwise, posting to /admin/settings/tunable. Rows show "modified
  from default" when the value differs from the compiled default.

types.go:
- Add SettingEntry (template-local mirror of database.SettingEntry,
  keeps templates from importing database) and SettingGroup.

utils.go:
- Add GroupTunableSettings: splits a flat, group-sorted entry list into
  live and restart []SettingGroup buckets preserving source order.
  utils_test.go covers the multi-group + empty cases.

frontend.go:
- The /admin/settings page handler now loads entries from the registry,
  drops the three keys that have dedicated UI cards (default_timezone
  dropdown, scan_poll_interval_seconds, auto_scan_enabled) so they are
  not listed twice, groups the rest, and passes liveGroups/restartGroups
  into the template.

templates/types.go

@@ -71,6 +71,28 @@ type ScanSettingsData struct {
 	ScanPollIntervalSeconds int
 }
 
+// SettingEntry mirrors database.SettingEntry for the admin UI. Kept as a
+// template-local type so the templates package does not import database.
+type SettingEntry struct {
+	Key             string
+	Value           string
+	Type            string
+	Min             string
+	Max             string
+	RequiresRestart bool
+	Category        string
+	Group           string
+	Description     string
+	IsDefault       bool
+}
+
+// SettingGroup is a labeled cluster of related settings rendered as a
+// sub-section within a tunable-settings card.
+type SettingGroup struct {
+	Name    string
+	Entries []SettingEntry
+}
+
 type SeriesCardData struct {
 	Name          string
 	BookCount     int64