feat(dashboard): serve H0M3 at /, relocate dashboard to /dashboard.html (#1464)

Step 2 of the nav restructure: make the H0M3 menu hub the landing page
at / and move the dashboard SPA to /dashboard.html, with every surface
linking back to the hub.

Mechanism (frontend-only, no host-side change — confirmed against the
ServeDir route table): the dashboard's ServeDir fallback serves
index.html at / via append_index_html_on_directories, and there is no
SPA path-routing catch-all to break. The dashboard SPA is served as the
plain file dashboard.html so it never shadows the exact-match
/dashboard/stream + /dashboard/history SSE routes registered before the
fallback.

- Swap the HTML entry files: the H0M3 page becomes index.html (loads
  home.js → served at /), and the dashboard SPA becomes dashboard.html
  (loads tabs.js → served at /dashboard.html). build.mjs copies the new
  set; JS bundle names are unchanged (referenced by absolute /static/
  paths, independent of the HTML filename).
- H0M3 Dashboard tile now points at /dashboard.html.
- The dashboard gains a "← home" back-link in its chrome; flow.html and
  logs.html relabel their back-link from "← dashboard" to "← home"
  (href stays / — which is the hub now). Pages link to the hub, not to
  each other.
- Agent page (app.js + stats.js): the "↑ dashboard" link now targets
  /dashboard.html. The API base (rebuild / answer-question /
  mark-all-read POSTs) stays the origin root, unchanged.
- Comment-only: tabs.js / flow.js / common.js references to the
  dashboard's old index.html filename updated to dashboard.html.

Note for review: git renders the file swap as a deleted home.html + an
added dashboard.html + a heavily-modified index.html, because index.html
exists on both sides with swapped content. It's a content swap, not a
rewrite — the built dist/ is verified (index.html→home.js, dashboard.html
→tabs.js).

Deferred to a follow-up: removing the FL0W / L0GS / M4TR1X "→" page-links
from the dashboard tab strip (touches tabs.js gating/overflow), and the
shared reusable chrome component (a later step).

frontend/packages/dashboard/src/flow.js

@@ -7,7 +7,7 @@
 //
 // Does NOT contain the dashboard's tab renderers, mutation-event
 // dispatchers, or refreshState — that's `./tabs.js`, loaded only by
-// /index.html. The flow page runs purely on the broker stream + an
+// /dashboard.html. The flow page runs purely on the broker stream + an
 // initial /api/state fetch (compose autocomplete needs the live
 // container list).
 
@@ -24,7 +24,7 @@ import {
 
   // ─── local containers cache (for compose autocomplete) ──────────────────
   // The compose box's @-mention completion suggests known agent names.
-  // /index.html (tabs.js) maintains the canonical `containersState`
+  // /dashboard.html (tabs.js) maintains the canonical `containersState`
   // from /api/state + SSE; here we keep a small local mirror updated
   // by the same `container_state_changed` / `container_removed` events
   // the dashboard would handle.
@@ -248,7 +248,7 @@ import {
         // Drop every other mutation kind silently — without this
         // they'd fall through to the terminal module's default
         // renderer and clutter the log with JSON dumps. The dashboard
-        // tabs handle these on /index.html via tabs.js.
+        // tabs handle these on /dashboard.html via tabs.js.
         _default: () => {},
       },
       // Re-sync the local containers cache on every SSE (re)connect.