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/TerminalModal.css

@@ -1,9 +1,16 @@
 /* === Terminal Modal === */
+/* Center vertically (override the overlay's default top alignment). */
+.modal-overlay:has(.terminal-modal) {
+  align-items: center;
+  padding-top: 0;
+  padding-bottom: 0;
+}
+
 .modal.terminal-modal {
   width: 90vw;
   max-width: 1600px;
-  min-height: 90vh;
-  max-height: 90vh;
+  min-height: 80vh;
+  max-height: 85vh;
   background: var(--card);
   display: flex;
   flex-direction: column;