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>

templates/partials/app-confirm-dialog.phtml

@@ -7,7 +7,7 @@
   aria-labelledby="app-confirm-dialog-title"
   aria-describedby="app-confirm-dialog-message"
 >
-  <article class="app-confirm-dialog">
+  <article class="app-confirm-dialog app-dialog-sm">
     <header>
       <h2 id="app-confirm-dialog-title" data-app-confirm-title><?php e(t('Please confirm')); ?></h2>
       <button type="button" class="close" data-app-confirm-close aria-label="<?php e(t('Close')); ?>" title="<?php e(t('Close')); ?>"></button>

templates/partials/app-session-warning-dialog.phtml

@@ -5,7 +5,7 @@
   aria-describedby="app-session-warning-message"
   aria-live="assertive"
 >
-  <article class="app-session-warning-dialog">
+  <article class="app-session-warning-dialog app-dialog-sm">
     <header>
       <h2 id="app-session-warning-title"><?php e(t('Session expiring')); ?></h2>
     </header>