feat(web/ui): consolidate dialogs, cards, and breakpoints (Pass 3)

- Unify modal/dialog system; .modal-* are now aliases of .dialog-*
- Migrate WorkspacesPage, start-tool-modal, merge-dialog, workspace-tools-panel to .dialog-*
- Add .card-sm/.card-md/.card-lg/.card-elevated/.card-borderless modifiers
- Apply card utilities across workspaces, projects, ssh-keys, workspace-detail, git-history
- Remove duplicated card-like background/border/padding from page CSS
- Remove 860px breakpoint; standardize on 767px/768px mobile split
- Add docs/development/ui-review-checklist.md
- Archive web-ui-spacing-typography-rework OpenSpec change

apps/web/src/components/features/git/merge-dialog.tsx

@@ -61,11 +61,13 @@ export const MergeDialog = ({
   };
 
   return (
-    <div className="modal-overlay" onClick={onClose}>
-      <div className="modal-content" onClick={(e) => e.stopPropagation()}>
-        <h2>Merge Branch</h2>
+    <div className="dialog-overlay" onClick={onClose}>
+      <div className="dialog" onClick={(e) => e.stopPropagation()}>
+        <div className="dialog-header">
+          <h3>Merge Branch</h3>
+        </div>
         
-        <div className="merge-form">
+        <div className="dialog-body merge-form">
           <div className="form-field">
             <label>Source Branch (merge from)</label>
             <select
@@ -108,7 +110,7 @@ export const MergeDialog = ({
             <div className="success-text">Merge successful!</div>
           )}
 
-          <div className="modal-actions">
+          <div className="dialog-actions">
             <button
               className="secondary-button"
               onClick={onClose}