frontend: cut over Rust binaries to ServeDir; delete legacy assets

Phase 4 of #273 — the actual switch. Both axum routers now serve
their static surface via `tower_http::services::ServeDir` mounted
as a fallback service, reading the dist path from `HIVE_STATIC_DIR`
(set by Phase 3's NixOS module wiring).

Deletes:
- `hive-c0re/assets/{index.html, app.js, dashboard.css}`
- `hive-ag3nt/assets/{index.html, app.js, agent.css, stats.html,
   stats.js, screen.html}`
- The whole `hive-fr0nt/` crate (workspace member dropped, both
  hive-c0re and hive-ag3nt drop their `hive-fr0nt.workspace = true`
  dep). Its contents now live as `@hive/shared` under
  `frontend/packages/shared/`.

Rust changes:
- `hive-c0re/src/dashboard.rs`: remove `serve_index`, `serve_css`,
  `serve_app_js`, `serve_shared_js`, `serve_marked_js`,
  `serve_favicon` (all six `include_str!` handlers); replace their
  routes with a single `.fallback_service(ServeDir::new(static_dir))`
  on the router. Fail closed (anyhow::bail) if `HIVE_STATIC_DIR` is
  unset or not a directory at startup.
- `hive-ag3nt/src/web_ui.rs`: remove `serve_index`, `serve_css`,
  `serve_app_js`, `serve_shared_js`, `serve_marked_js`,
  `serve_stats`, `serve_stats_js`, `serve_screen`; same
  `fallback_service` pattern. `serve_icon` stays (consumes
  `/etc/hyperhive/icon.svg` + `branding/hyperhive.svg` fallback,
  neither of which lives under the frontend dist).
- `AgentLink` URLs for stats/screen switched from `/stats` / `/screen`
  to `/stats.html` / `/screen.html` since ServeDir doesn't auto-
  append the extension and the on-disk filename is the natural URL
  post-cutover.
- `Cargo.toml` (workspace): drop `hive-fr0nt` member + workspace
  dep, add `tower-http = { version = "0.6", features = ["fs"] }`.
- `hive-c0re/Cargo.toml` + `hive-ag3nt/Cargo.toml`: drop the
  `hive-fr0nt.workspace = true` dep, add `tower-http.workspace =
  true`.

Docs updated:
- `CLAUDE.md`: file map reflects `frontend/` (was `hive-fr0nt/` +
  `assets/`) and the ServeDir/HIVE_STATIC_DIR shape.
- `docs/web-ui.md` 'Shape (shared by both)' section: describes the
  ServeDir fallback + bundled-by-esbuild surface, no more
  `include_str!` references.
- `docs/terminal-rendering.md`: src paths point at
  `frontend/packages/{agent,shared}/src/`; marked is the npm dep,
  not vendored UMD.

Validation:
- `cargo check --workspace` — clean (5 warnings, all pre-existing
  in `rebuild_queue.rs`, none on changed files).
- `cargo clippy --workspace --all-targets` — clean (11 warnings,
  same pre-existing source).
- `cd frontend && npm run build` from the prior commit's lockfile
  produces the dist directories the new routers consume:
    dashboard: `dist/{index.html, static/{app.js, dashboard.css}}`
    agent:     `dist/{index.html, stats.html, screen.html,
                       static/{app.js, stats.js, agent.css}}`
  (favicon.svg lands in dashboard/ during the nix build —
  `nix/frontend.nix` install phase copies `branding/hyperhive.svg`
  there, since it's outside the npm tree.)

Refs #273.

CLAUDE.md

@@ -87,35 +87,28 @@ hive-c0re/         host daemon + CLI (one binary, subcommand-dispatched)
                          and meta read access; mirrors each applied repo
                          into `agent-configs/<n>` (core-only); agents are
                          read-only collaborators on `core/meta`
-  src/dashboard.rs      axum HTTP: static shell + /api/state JSON + actions
+  src/dashboard.rs      axum HTTP: /api/state JSON + actions
                          + journald viewer + bind-with-retry (SO_REUSEADDR)
                          + deployed_sha chip per container +
                          /dashboard/{stream,history} subscribing to the
-                         unified DashboardEvent channel
-  assets/               index.html, dashboard.css, app.js (include_str!)
+                         unified DashboardEvent channel. Static assets
+                         (HTML/CSS/JS/favicon) served by
+                         tower_http::ServeDir from $HIVE_STATIC_DIR
+                         (= `${frontend}/dashboard` per the c0re module).
 
-hive-fr0nt/           shared frontend-assets crate (browser only).
-  src/lib.rs            pub const BASE_CSS / TERMINAL_CSS / TERMINAL_JS /
-                         MARKED_JS re-exports; both binaries
-                         `include_str!` them and prepend to their per-
-                         page serving routes.
-  assets/base.css       Catppuccin palette + body typography (one source
-                         of truth, no per-page redeclaration).
-  assets/terminal.css   `.terminal-wrap` + `.live` + `.tail-pill` +
-                         `.row` / `details.row` styling for both
-                         pages' lit log panes. Unified prefix-column
-                         (padding-left + negative text-indent) so glyph
-                         alignment is consistent across row kinds + a
-                         `.md` block scope for marked-rendered bodies.
-  assets/terminal.js    `window.HiveTerminal.create(opts)`: scroll-
-                         sticky log + "↓ N new" pill + history
-                         backfill + SSE subscribe-buffer-snapshot-
-                         dedupe dance. Pages register a kind→renderer
-                         map; the terminal owns the lifecycle.
-  assets/marked.umd.js  vendored marked v4.0.2 UMD bundle. Per-agent
-                         terminal uses the global `marked.parse` for
-                         markdown bodies on send / recv / ask / answer
-                         / assistant text rows.
+frontend/             npm workspaces (esbuild → static dist). Built
+                       hermetically by `nix/frontend.nix`
+                       (`packages.${system}.frontend`).
+  packages/shared/      @hive/shared: terminal pane + Catppuccin palette
+                         + base typography (was hive-fr0nt). ES module
+                         exporting { create, linkify }; pure JS, no IIFE
+                         globals; consumed by dashboard + agent.
+  packages/dashboard/   @hive/dashboard SPA: src/{index.html, app.js,
+                         dashboard.css} + build.mjs → dist/{index.html,
+                         static/{app.js, dashboard.css}}.
+  packages/agent/       @hive/agent default per-container UI: src/
+                         {index, stats, screen}.html + {app, stats}.js
+                         + agent.css → dist/{*.html, static/*}.
 
 hive-ag3nt/        in-container harness crate; produces TWO binaries
   src/lib.rs            re-exports + DEFAULT_SOCKET, DEFAULT_WEB_PORT
@@ -139,8 +132,10 @@ hive-ag3nt/        in-container harness crate; produces TWO binaries
   src/login_session.rs  drives `claude auth login` over stdio pipes
   src/bin/hive-ag3nt.rs sub-agent main (Serve + Mcp subcommands)
   src/bin/hive-m1nd.rs  manager main (Serve + Mcp subcommands)
-  assets/               index.html, agent.css, app.js, stats.html,
-                         stats.js, screen.html (include_str!)
+                         Static UI assets served by ServeDir from
+                         $HIVE_STATIC_DIR (= hyperhive.frontend
+                         .mergedDist — default agent dist + per-agent
+                         extraFiles, set per the harness-base module).
   prompts/              static role/tools/settings for claude (include_str!):
                           agent.md  — sub-agent system prompt
                           manager.md — manager system prompt