feat(dashboard): polish modal widths, popover usage, onboarding contrast, TUI logo

- Resize the dashboard TUI splash logo to scale with the terminal: a Colossal-font variant kicks in on terminals ≥70 cols × 16 rows; gradient palette is now strictly white → blueBright → blue (no cyan/magenta)
- Lower-clip TUI Kanban cards to a single-line title and constrain each column with overflow hidden so cards no longer bleed past the header row
- Center the TerminalModal vertically by overriding overlay alignment via :has() and trimming the modal height
- Match GitHub Import modal width to the file browser modal (90vw / 1600px max)
- Render the Usage indicator as a top-right popover on desktop instead of a centered modal (mobile keeps the full-screen sheet)
- Widen SetupWizard / Model Onboarding modals (540→880px, 560→880px) and replace the washed-out done step indicator background with a solid success swatch
- Drop the duplicate mailbox icon button from the desktop header (view switch covers it; overflow menu still surfaces it on compact)
- Loosen agent health detection: bump the heartbeat grace multiplier 2× → 4× and the staleness floor 60s → 5min so transient ticks don't flag agents Unresponsive

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

packages/dashboard/app/components/PlanningModeModal.css

@@ -1634,3 +1634,20 @@
   }
 }
 
+/* Desktop: render as a top-right popover card instead of a centered modal */
+@media (min-width: 769px) {
+  .modal-overlay:has(.usage-modal) {
+    background: transparent;
+    backdrop-filter: none;
+    align-items: flex-start;
+    justify-content: flex-end;
+    padding: 60px var(--space-md) 0 0;
+  }
+
+  .modal.usage-modal {
+    width: 420px;
+    max-height: 70vh;
+    box-shadow: var(--shadow-lg);
+  }
+}
+