announce server hostname in MANIFEST, display in client

Server reads its hostname via gethostname(2) and includes it as a
length-prefixed UTF-8 string at the start of the MANIFEST payload.

Client displays 'Server: <hostname> — <MAC>' in a label above the
upstream list. Both sides updated for the new MANIFEST format:
  [hostname_len:1][hostname:N][entries...]

Protocol version unchanged (still 1); MANIFEST payload layout change
is backward-incompatible but both sides ship together.

PROTOCOL.md

@@ -59,18 +59,27 @@ Reserved (unimplemented in v1; parse returns Err, encode unimplemented):
 
 ### MANIFEST payload
 
-Variable-length entries, parsed sequentially until the payload is consumed.
+A hostname prefix followed by variable-length entries, parsed sequentially
+until the payload is consumed.
 
 ```
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
-+---------------+---------------+-------------------------------+
++---------------+-----------------------------------------------+
+| hostname_len  | hostname (UTF-8, hostname_len bytes) ...       |
++---------------+-----------------------------------------------+
 | id            | proto         | port (big-endian)             |
 +---------------+---------------+-------------------------------+
 | label_len     | label (UTF-8, label_len bytes) ...           |
 +---------------+-----------------------------------------------+
+| ... repeated ...                                             |
++---------------------------------------------------------------+
 ```
 
+- **hostname_len** (u8): length in bytes of the server's hostname. `0` is valid
+  (unknown hostname).
+- **hostname** (`hostname_len` bytes, UTF-8): the server's hostname, read via
+  `gethostname(2)` at startup. Maximum 255 bytes.
 - **id** (u8): upstream identifier (1-based positional index from `gatunad`
   cmdline).
 - **proto** (u8): `1 = TCP`, `2 = UDP` (reserved; not emitted in v1).
@@ -80,7 +89,8 @@ Variable-length entries, parsed sequentially until the payload is consumed.
 - **label** (`label_len` bytes, UTF-8): human-readable name for the upstream,
   taken from the `PORT[:label]` cmdline argument. Maximum 255 bytes.
 
-To parse: read the 5-byte fixed prefix, then `label_len` bytes, and repeat until
+To parse: read `hostname_len`, then `hostname_len` bytes of hostname, then read
+5-byte fixed entry prefixes + `label_len` bytes of label each, repeating until
 the payload is exhausted. The number of entries is not carried explicitly.
 
 ### OPEN payload