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;

web/css/components/app-dialog.css

@@ -5,6 +5,18 @@
  * Variant-specific styles live in their own component files (app-confirm-dialog.css, etc.).
  */
 @layer components {
+  /* ── Size tiers ────────────────────────────────────────────────────────
+   * Fixed-width tiers so every dialog of the same purpose feels consistent.
+   * Apply one of these classes to the <article> inside <dialog>.
+   *   sm (24rem) — simple prompts, confirmations, alerts
+   *   md (32rem) — form dialogs with 1-3 fields, pickers
+   *   lg (40rem) — complex forms, multi-section dialogs
+   * min() ensures graceful shrink on narrow viewports.
+   * ──────────────────────────────────────────────────────────────────── */
+  dialog > article.app-dialog-sm { width: min(24rem, calc(100vw - 2rem)); }
+  dialog > article.app-dialog-md { width: min(32rem, calc(100vw - 2rem)); }
+  dialog > article.app-dialog-lg { width: min(40rem, calc(100vw - 2rem)); }
+
   /* ── Footer layout ───────────────────────────────────────────────────── */
   dialog > article > footer,
   dialog > article form > footer {