dashboard: tab-bar restructure + extract FL0W to /flow.html (#369)

Operator: 'option A (tabs)' (#369#issuecomment-3434) +
'yes terminal can be a separate page' (#369#issuecomment-3437).

## Tab framework

`index.html` becomes a 3-tab dashboard with a sticky chrome header:

- `◆ SW4RM ◆`    — containers list (the central thing)
- `◆ Y3R C4LL ◆` — pending approvals + operator-targeted questions
- `◆ SYST3M ◆`   — meta inputs + rebuild queue + reminders + tombstones

Hash routing: `#swarm` / `#call` / `#system` (empty → SW4RM).
F5-reloadable + back-button-aware without a router framework.

SSE stays alive across tab switches — count pills on inactive tabs
update live so the operator never loses pulse on what's happening
elsewhere:

  - SW4RM:  containers with needs_update
  - Y3R C4LL: approvals.pending + questions.pending (attn-coloured pill)
  - SYST3M: rebuild_queue entries in Queued|Running

Pills hidden when count is zero. setInterval(1s) polls the existing
state stores (cheap, no per-renderer hookup needed).

## FL0W as its own page

The all-agents chat moves to /flow.html — full-viewport vibec0re
layout mirroring the per-agent live page (#362):

- Fixed-overlay frosted-glass header at top (back link + title +
  notif controls), backdrop-filter blur shows the scrolled chat
  text behind.
- Full-viewport terminal, scroll-padded for the floating chrome so
  first/last rows stay reachable.
- Fixed-overlay frosted composer at the bottom.
- Operator inbox surfaces via a pill (📬 inbox · N) in the upper
  right — click opens the side-panel flyout with the message list.

In the dashboard tab strip, FL0W is the right-most entry but
renders as a `<a class="tab tab-link" href="/flow.html">` — clicking
navigates to the page rather than swapping a pane. Same pattern
back from flow.html via the `← d4shb04rd` link.

## Implementation notes

- New `/flow.html` page rendered by the same bundled `app.js` — the
  flow page just doesn't have the dashboard-chrome DOM, so the
  matching renderers no-op silently (each `if (!el) return`).
  Avoids splitting the bundle for v1; can extract later if size
  becomes a concern.
- `Panel` module gains `openNamed(name, …)` + `refresh(name, …)` —
  the legacy untyped `open(title, content)` calls clear the owner,
  so file-preview / diff / log drill-ins behave unchanged. `refresh`
  is no-op when a different view owns the panel, so live message
  events re-render the inbox flyout only when it's actually open.
- `renderInbox` updates BOTH the dashboard's inline `#inbox-section`
  (now living on the flow page) AND the flow page's pill count +
  side-panel refresh. The dashboard's empty FL0W tab is removed —
  inbox + message flow + compose box only exist in flow.html.
- Banner shrinks to a thin Catppuccin gradient strip at the top of
  the dashboard chrome (dropped the multi-line ASCII art —
  affectionate but pure chrome budget in a tabbed layout).
- `build.mjs` copies both `index.html` + `flow.html` into dist.

## Validation

`npm run build` clean. Dashboard bundle deltas:
  app.js  150kb → 152kb  (tab routing + count pills + named-Panel)
  dashboard.css 33kb → 38kb (tab chrome + flow page layout)
  + dist/flow.html  4.4kb

Browser smoke test isn't possible from inside iris's container
(no JS engine) — drafting as a PR for operator visual review on
next deploy. Worth eyeballing:
  - Tab switching feels right; counts update live across SSE events
  - FL0W page reads like the agent live page (frosted header + composer)
  - Inbox pill opens flyout; live message arrivals refresh it
  - Back link from flow → dashboard returns to last tab via the
    URL hash (browser remembers the hash across page nav)

Closes #369.

frontend/packages/dashboard/build.mjs

@@ -48,6 +48,8 @@ await build({
   logLevel: 'info',
 });
 
-copyFileSync(src('index.html'), dist('index.html'));
+for (const html of ['index.html', 'flow.html']) {
+  copyFileSync(src(html), dist(html));
+}
 
 console.log('dashboard build ok →', dist(''));