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/build.mjs

@@ -1,29 +1,38 @@
 // esbuild build for @hive/dashboard. Output layout (`dist/`):
 //
-//   dist/index.html               served by the Rust router at GET /
+//   dist/index.html               H0M3 menu hub — served by the Rust
+//                                  router at GET / (the landing page)
+//   dist/dashboard.html           the operator dashboard SPA — served
+//                                  at GET /dashboard.html
 //   dist/flow.html                served at GET /flow.html
 //   dist/logs.html                served at GET /logs.html
-//   dist/static/tabs.js           /index.html entry — tab renderers +
+//   dist/static/home.js           index.html (H0M3) entry — menu tiles +
+//                                  matrix-tile gating + identity line
+//   dist/static/tabs.js           dashboard.html entry — tab renderers +
 //                                  tab routing + refreshState
 //   dist/static/flow.js           /flow.html entry — broker terminal +
-//                                  operator inbox + @-mention composer
+//                                  @-mention composer
 //   dist/static/logs.js           /logs.html entry — build/agent/system
 //                                  log viewer sub-tabs
-//   dist/static/{tabs,flow,logs}.js.map source map siblings
+//   dist/static/{home,tabs,flow,logs}.js.map source map siblings
 //   dist/static/common.css        loaded by every page (@hive/shared
 //                                  imports + shared typography/badges/
 //                                  buttons/inbox/side-panel)
-//   dist/static/dashboard.css     /index.html only (dashboard-specific)
+//   dist/static/dashboard.css     dashboard.html only (dashboard-specific)
 //   dist/static/flow.css          /flow.html only (flow chrome + composer)
 //   dist/static/logs.css          /logs.html only (log viewer sub-tabs)
+//   dist/static/home.css          index.html (H0M3) only (tile grid)
 //
-// All three JS entries bundle `./common.js` (DOM helpers, Panel singleton,
+// Each JS entry bundles `./common.js` (DOM helpers, Panel singleton,
 // NOTIF, path linkification) independently — esbuild inlines the shared
 // module into each bundle rather than emitting a shared chunk (no
 // `splitting: true`). The Rust binary mounts `dist/` as a
-// `tower_http::ServeDir` fallback; the layout above keeps every URL
-// the HTML files reference reachable without rewriting paths in the
-// HTML.
+// `tower_http::ServeDir` fallback (`append_index_html_on_directories`
+// serves index.html at /); the layout above keeps every URL the HTML
+// files reference reachable without rewriting paths in the HTML. The
+// dashboard SPA lives at /dashboard.html (a plain file, not a /dashboard/
+// prefix) so it never shadows the exact-match /dashboard/stream +
+// /dashboard/history SSE routes registered before the ServeDir fallback.
 
 import { build } from 'esbuild';
 import { mkdirSync, copyFileSync, rmSync } from 'node:fs';
@@ -91,7 +100,7 @@ for (const entry of ['theme.css', 'common.css', 'dashboard.css', 'flow.css', 'lo
   });
 }
 
-for (const html of ['index.html', 'flow.html', 'logs.html', 'home.html']) {
+for (const html of ['index.html', 'dashboard.html', 'flow.html', 'logs.html']) {
   copyFileSync(src(html), dist(html));
 }