feat(ui): standardize dialog widths with tiered size system (sm/md/lg)

Add three reusable size tier classes (app-dialog-sm, app-dialog-md,
app-dialog-lg) to base dialog CSS, replacing per-variant max-width
rules. Each tier uses width: min(Xrem, calc(100vw - 2rem)) for
consistent fixed widths with responsive mobile fallback.

- Confirm dialog + session warning: app-dialog-sm (24rem)
- Bookmark + bookmark group dialog: app-dialog-md (32rem)
- Remove max-width from app-confirm-dialog.css and app-bookmark-form.css

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/components/app-confirm-dialog.css

@@ -1,9 +1,5 @@
 @layer components {
   /* Confirm dialog — modal overlay for destructive action confirmation with danger/warning variants. */
-  dialog[data-app-confirm-dialog] > article.app-confirm-dialog {
-    max-width: 32rem;
-  }
-
   dialog[data-app-confirm-dialog] > article.app-confirm-dialog > header {
     position: relative;
     padding-inline-end: 2rem;