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/SetupWizardModal.css

@@ -29,8 +29,8 @@
 .setup-wizard-modal {
   display: flex;
   flex-direction: column;
-  max-width: 540px;
-  width: 100%;
+  max-width: 880px;
+  width: 90vw;
   max-height: min(720px, calc(100dvh - var(--overlay-padding-top, 10vh) - var(--space-lg)));
   animation: slideUp var(--transition-normal) ease-out;
   overflow: hidden;
@@ -475,7 +475,7 @@
 /* ===== Model Onboarding Modal ===== */
 
 .model-onboarding-modal {
-  max-width: 560px;
+  max-width: 880px;
   width: 90vw;
   max-height: 85vh;
   display: flex;
@@ -565,9 +565,9 @@
 }
 
 .model-onboarding-step-indicator.done .step-number {
-  background: var(--status-done-bg-deep);
-  border: 1px solid color-mix(in srgb, var(--done) 45%, transparent);
-  color: var(--done);
+  background: var(--color-success);
+  border: 1px solid var(--color-success);
+  color: #fff;
 }
 
 .model-onboarding-step-indicator.done {