flow: keep the dashboard tab strip on the flow page (#383)

Operator wanted the tab header visible on /flow.html so switching
tabs doesn't require navigating back to / first.

The flow page now reuses the same `<header class="dashboard-chrome">`
markup the dashboard renders, with a few tweaks:

- The SW4RM / Y3R C4LL / SYST3M tabs are cross-page links
  (`href="/#swarm"` etc.) — clicking lands on the dashboard with the
  destination tab pre-active via the hash router.
- The FL0W tab is rendered `.active.tab-link` + `aria-current="page"`
  so it reads as the current view (no clickable arrow / "go here"
  affordance — you're already here).
- Banner-thin echoes the dashboard for visual continuity.
- Notif controls cohabit with the tabs (same IDs the dashboard uses,
  so app.js's NOTIF binding picks them up unchanged).

Layout glue:
- `body.flow-shell .dashboard-chrome.flow-chrome` overrides the
  dashboard's `position: sticky` with `position: fixed` so the
  chrome stays put under flow-shell's `overflow: hidden` body
  layout, keeping the terminal full-viewport behind/beneath.
- New rule for the active FL0W tab — the `.tab-link` styling on the
  dashboard otherwise reads as a passive cross-page link; here we
  need it lit-up like a regular active tab.
- `--flow-header-h` bumped from 4.2em → 4.7em to match the natural
  height of the tab strip + banner combo. Terminal padding +
  inbox-pill top offset both derive from this variable, so they
  follow automatically.

Removed:
- Legacy `.flow-title`, `.flow-hint`, `.flow-back` CSS rules (their
  HTML counterparts are gone — the tab strip carries the
  identity now).
- The `<a class="flow-back">← d4shb04rd</a>` link and the
  `<h2 class="flow-title">` from flow.html.

## Validation

`npm run build` clean.
  dashboard.css: 38kb → 37kb (legacy rules removed, new shared-
                  chrome rules are smaller)
  flow.html:    4.4kb → 4.7kb (tab strip replaces title bar)
  app.js:       unchanged (no JS changes — the tab navigation is
                pure HTML href + cross-page hash)

Closes #383.

frontend/packages/dashboard/src/dashboard.css

@@ -1290,7 +1290,12 @@ footer a { color: var(--purple); }
    ergonomics without stealing terminal real estate. */
 
 :root {
-  --flow-header-h: 4.2em;
+  /* Approximate height of the flow chrome (banner-thin + tabbar +
+     dashboard-chrome padding). Bumped slightly above the natural
+     content height so the terminal padding-top clears the bottom
+     border. Tweaked once after #383 took over the chrome from the
+     simpler title bar. */
+  --flow-header-h: 4.7em;
   --flow-composer-h: 3.6em;
   --flow-frost-bg: rgba(30, 30, 46, 0.74);
   --flow-frost-blur: blur(12px) saturate(140%);
@@ -1311,58 +1316,40 @@ body.flow-shell {
     var(--bg);
 }
 
-.flow-header {
+/* Flow chrome (#383): reuses the dashboard's `.dashboard-chrome` +
+   tabbar so the operator can switch tabs from the flow page without
+   navigating back first. The chrome must be fixed-position (vs
+   sticky on the dashboard) since flow-shell has `overflow: hidden`
+   on body and the main area absolute-positions the terminal. */
+body.flow-shell .dashboard-chrome.flow-chrome {
   position: fixed;
   top: 0;
   left: 0;
   right: 0;
   z-index: 30;
-  min-height: var(--flow-header-h);
-  display: flex;
-  align-items: center;
-  gap: 1em;
-  padding: 0.55em 1em;
+  margin: 0;
+  padding: 0.4em 0 0;
   background: var(--flow-frost-bg);
   -webkit-backdrop-filter: var(--flow-frost-blur);
   backdrop-filter: var(--flow-frost-blur);
   border-bottom: 1px solid var(--purple-dim);
   box-shadow: 0 6px 18px rgba(0, 0, 0, 0.35);
-  flex-wrap: wrap;
 }
-.flow-back {
-  color: var(--cyan);
-  text-decoration: none;
-  font-size: 0.85em;
-  letter-spacing: 0.1em;
-  padding: 0.25em 0.6em;
-  border: 1px solid var(--purple-dim);
-  border-radius: 4px;
-  flex: 0 0 auto;
-  transition: border-color 0.15s ease, color 0.15s ease;
-}
-.flow-back:hover {
-  border-color: var(--cyan);
-  text-shadow: 0 0 8px rgba(137, 220, 235, 0.6);
-}
-.flow-title {
-  margin: 0;
-  font-size: 1.2em;
+/* Active tab on the flow page is FL0W. Its `.tab-link` styling (the
+   tab strip's right-most entry on the dashboard, where it represents
+   the *link out* to /flow.html) needs to read as the active tab here.
+   `aria-current="page"` flags it semantically. */
+body.flow-shell .tabbar .tab.active.tab-link {
   color: var(--purple);
-  text-transform: uppercase;
-  letter-spacing: 0.15em;
-}
-.flow-hint {
-  margin: 0;
-  color: var(--muted);
-  font-size: 0.82em;
-  flex: 1 1 auto;
-  min-width: 0;
-}
-.flow-header .notif-row {
-  margin-left: auto;
-  display: flex;
-  gap: 0.4em;
+  background: var(--bg);
+  border-color: var(--purple-dim);
+  box-shadow: 0 -2px 12px -4px rgba(203, 166, 247, 0.4);
 }
+/* Legacy `.flow-title` / `.flow-hint` / `.flow-back` rules were
+   removed in #383 — the flow page now uses the shared chrome with
+   the dashboard tab strip, no need for FL0W-specific title/hint
+   elements. The `.notif-row` styling lives under the shared
+   `.tabbar #notif-row` selector earlier in the file. */
 
 /* Inbox pill — operator inbox flyout trigger. Sits right under the
    header so it stays in the operator's gaze without crowding the