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/dashboard.css

@@ -30,6 +30,19 @@ body.dashboard-shell {
   padding: 0.4em 0 0;
   margin: 0 -1.5em 1em;
 }
+/* ← home back-link to the H0M3 hub. Small purple link top-left of the
+   chrome, mirroring the .logs-back treatment the sub-pages use so the
+   navigation reads consistently. Full chrome unification (a shared,
+   reusable tab/back-link component) is a later step. */
+.dash-home-back {
+  display: inline-block;
+  color: var(--purple);
+  text-decoration: none;
+  font-size: 0.82em;
+  padding: 0.1em 1em 0;
+  white-space: nowrap;
+}
+.dash-home-back:hover { text-decoration: underline; }
 .banner-thin {
   text-align: center;
   margin: 0;