fix(dashboard): mobile rendering for resizable modals

The Settings, Workflow Steps, Automations (and friends) modals all carried
desktop `min-width: 480-520px` constraints from the resize work. On phones
that pushed them off-screen, and the overlay's 10vh top padding plus a
fractional `height: 80vh` left awkward gaps top and bottom.

Apply the same `@media (max-width: 768px)` full-screen-sheet treatment
that File Browser already had to every resize-aware modal: drop the
overlay padding, set width/height to `100vw`/`100dvh`, zero out the
min-* / max-* constraints, and disable `resize` (touchscreens can't grab
the corner grip anyway).

Also tightened the existing File Browser and GitHub Import overrides to
match (`min-width: 0`, `min-height: 0`, `resize: none`).

For TaskDetailModal the mobile rules go inside the existing first
`@media (max-width: 768px)` block to keep the FN-1331 detail-body
padding regression test happy (it captures from the first 768px media
query greedily).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/WorkflowStepManager.css

@@ -408,15 +408,25 @@
 }
 
 @media (max-width: 768px) {
-  .workflow-step-manager-modal {
-    width: 100%;
-    max-width: 100%;
-    height: 100vh;
+  /* Full-screen sheet on mobile — drop the overlay's top padding so the
+     modal fills the viewport instead of being pushed below it. */
+  .modal-overlay:has(.workflow-step-manager-modal) {
+    padding-top: 0;
+    align-items: stretch;
+    justify-content: stretch;
+  }
+
+  .modal.workflow-step-manager-modal {
+    width: 100vw;
+    min-width: 0;
+    max-width: 100vw;
     height: 100dvh;
-    max-height: 100vh;
+    min-height: 0;
     max-height: 100dvh;
-    border-radius: 0;
+    margin: 0;
     border: none;
+    border-radius: 0;
+    resize: none;
   }
 
   .wfm-body {