feat(ui): Charcoal theme, elegant selection, rounded note list, confirm sign-out

- A new Charcoal theme — neutral dark greys on the menu, note list and
  editor instead of the default's faint green cast, with matching inks;
  the green accent stays. It joins the theme picker on its own (the picker
  is data-driven from THEMES).
- The text selection is a translucent wash of the accent, not the old
  near-invisible tint and not a solid slab — visible on white and on the
  dark surfaces, keeping the text's own colour.
- The note list is a rounded card now, the same corner and border as the
  editor beside it, with a few pixels more air around the menu and the list.
- Signing out asks first: a second click within a few seconds confirms, so
  a button the pointer passes over does not drop you at the login screen.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/stores.svelte.js

@@ -1006,6 +1006,7 @@ export const completedView = new CompletedView();
  */
 export const THEMES = [
 	{ id: 'dark', name: 'theme.dark', note: 'theme.darkNote', dark: true },
+	{ id: 'charcoal', name: 'theme.charcoal', note: 'theme.charcoalNote', dark: true },
 	{ id: 'dusk', name: 'theme.dusk', note: 'theme.duskNote', dark: true },
 	{ id: 'light', name: 'theme.light', note: 'theme.lightNote', dark: false },
 	{ id: 'paper', name: 'theme.paper', note: 'theme.paperNote', dark: false },