Add theme color editor and multi-step undo

- New /edit/theme-settings page with native color pickers for the
  background/primary/positive/negative theme colors plus light mode,
  contrast and saturation multipliers, custom-css-file path, and
  picker-disable. Pickers use hex; the backend converts to Glance's
  HSL "H S L" form before writing. Theme presets and full custom CSS
  intentionally still YAML.
- Numbered backup chain: save() rotates up to 10 prior versions as
  glance.yml.bak.1 .. .10 instead of a single .bak. The /edit page
  lists them with humanized timestamps and a per-row Restore button.
  Single-step "Restore previous (toggle)" is preserved.
- Restoring any backup saves the just-current state as a fresh
  .bak.1, so undo-the-undo always works.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/glance/glance.go

@@ -456,6 +456,7 @@ func (a *application) server() (func() error, func() error) {
 	mux.HandleFunc("GET /edit/pages/{page}", a.handleAdminPage)
 	mux.HandleFunc("GET /edit/pages/{page}/settings", a.handleAdminPageSettings)
 	mux.HandleFunc("GET /edit/site-settings", a.handleAdminSiteSettings)
+	mux.HandleFunc("GET /edit/theme-settings", a.handleAdminThemeSettings)
 	mux.HandleFunc("GET /edit/pages/{page}/widgets/{col}/{idx}", a.handleAdminWidget)
 
 	mux.HandleFunc("POST /edit/api/pages", a.handleAdminAddPage)
@@ -479,7 +480,9 @@ func (a *application) server() (func() error, func() error) {
 	mux.HandleFunc("POST /edit/api/pages/{page}/fields", a.handleAdminUpdatePageFields)
 	mux.HandleFunc("POST /edit/api/pages/{page}/move", a.handleAdminMovePage)
 	mux.HandleFunc("POST /edit/api/restore", a.handleAdminRestore)
+	mux.HandleFunc("POST /edit/api/restore/{n}", a.handleAdminRestoreFromBackup)
 	mux.HandleFunc("POST /edit/api/site-settings", a.handleAdminUpdateSiteSettings)
+	mux.HandleFunc("POST /edit/api/theme-settings", a.handleAdminUpdateTheme)
 
 	if a.RequiresAuth {
 		mux.HandleFunc("GET /login", a.handleLoginPageRequest)