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.

Cargo.lock

@@ -565,7 +565,6 @@ dependencies = [
  "axum",
  "clap",
  "futures-util",
- "hive-fr0nt",
  "hive-sh4re",
  "reqwest",
  "rmcp",
@@ -575,6 +574,7 @@ dependencies = [
  "serde_json",
  "tokio",
  "tokio-stream",
+ "tower-http",
  "tracing",
  "tracing-subscriber",
 ]
@@ -587,7 +587,6 @@ dependencies = [
  "axum",
  "base64",
  "clap",
- "hive-fr0nt",
  "hive-sh4re",
  "libc",
  "reqwest",
@@ -597,14 +596,11 @@ dependencies = [
  "tempfile",
  "tokio",
  "tokio-stream",
+ "tower-http",
  "tracing",
  "tracing-subscriber",
 ]
 
-[[package]]
-name = "hive-fr0nt"
-version = "0.1.0"
-
 [[package]]
 name = "hive-sh4re"
 version = "0.1.0"
@@ -645,6 +641,12 @@ dependencies = [
  "pin-project-lite",
 ]
 
+[[package]]
+name = "http-range-header"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9171a2ea8a68358193d15dd5d70c1c10a2afc3e7e4c5bc92bc9f025cebd7359c"
+
 [[package]]
 name = "httparse"
 version = "1.10.1"
@@ -954,6 +956,16 @@ version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6877bb514081ee2a7ff5ef9de3281f14a4dd4bceac4c09388074a6b5df8a139a"
 
+[[package]]
+name = "mime_guess"
+version = "2.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f7c44f8e672c00fe5308fa235f821cb4198414e1c77935c1ab6948d3fd78550e"
+dependencies = [
+ "mime",
+ "unicase",
+]
+
 [[package]]
 name = "mio"
 version = "1.2.0"
@@ -1723,10 +1735,19 @@ checksum = "4cfcf7e2740e6fc6d4d688b4ef00650406bb94adf4731e43c096c3a19fe40840"
 dependencies = [
  "bitflags",
  "bytes",
+ "futures-core",
  "futures-util",
  "http",
  "http-body",
+ "http-body-util",
+ "http-range-header",
+ "httpdate",
+ "mime",
+ "mime_guess",
+ "percent-encoding",
  "pin-project-lite",
+ "tokio",
+ "tokio-util",
  "tower",
  "tower-layer",
  "tower-service",
@@ -1835,6 +1856,12 @@ version = "1.20.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "40ce102ab67701b8526c123c1bab5cbe42d7040ccfd0f64af1a385808d2f43de"
 
+[[package]]
+name = "unicase"
+version = "2.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dbc4bc3a9f746d862c45cb89d705aa10f187bb96c76001afab07a0d35ce60142"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"