contact messages: visitor identity, relative timestamps, alternating headers

- The first message of a thread must carry name + email introducing the
  visitor; the panel shows the two fields until the thread has an
  identity (send stays disabled while they look empty, so the draft is
  never lost), prefilled from and saved to localStorage. The identity
  is stored on ContactThread (msgspec defaults — old databases decode
  unchanged) and fixed afterwards; it labels the visitor's messages
  instead of 'you', and the admin inbox lists threads by name with the
  email as a mailto in the thread header.
- Timestamps use the analytics relative formatter (formatWhen), with
  the full date on hover.
- Message headers alternate alignment (own messages right, theirs
  left) as the visual side cue; the sent-from page link is dropped
  from the display (kept as stored metadata).
- Composer gains a sendDisabled prop for the intro gate.

docs/messages.md

@@ -49,12 +49,14 @@ no extra connection). Frames **to** the server:
   `pagerite.js` re-sends this on every socket (re)connect, which is the
   resync mechanism (the socket idle-closes after 5 min and reconnects on
   activity).
-- `{token, msg, pow, fr}` — append a message; `msg` is the Markdown-subset
-  source, `pow` the solved proof-of-work (below), `fr` the page path it
-  was sent from (stored for context).
+- `{token, msg, pow, fr, name?, email?}` — append a message; `msg` is the
+  Markdown-subset source, `pow` the solved proof-of-work (below), `fr` the
+  page path it was sent from (stored for context). The **first** message
+  of a thread must carry `name` and `email` introducing the visitor; the
+  identity is stored on the thread and fixed from then on.
 
-Frames **from** the server: `{type: "thread", id, created, unread,
-messages: [{id, t, admin, html, page}]}` — a full thread snapshot
+Frames **from** the server: `{type: "thread", id, created, unread, name,
+email, messages: [{id, t, admin, html, page}]}` — a full thread snapshot
 (replace/upsert by `id`); `{type: "pow", challenge, required}` — a fresh
 single-use PoW challenge (`required` means the last message was rejected
 for a missing/stale solution and should be re-sent once solved); or
@@ -142,18 +144,18 @@ server restart (the pending set is RAM-only) still linger — no GC.
 
 `Data.contact: dict[bytes, ContactThread]` in the kanta database, keyed by
 the 12-byte blake3 hash of the session token (never the token itself). A
-thread is `created`, `admin_read`, and `messages: dict[str,
-ContactMessage]` keyed by a sortable timestamp-random id (dict-keyed, so a
-save's database diff touches only the appended message). Each message
-stores `t`, the Markdown source `md`, its strict rendering `html`, `admin`
-("" = visitor, else the SSO name) and `page`. Appends are single
-transactions (`contact` / `contact:reply` / `contact:read`).
+thread is `created`, `admin_read`, the visitor's self-entered
+`name`/`email` (required with the first message, fixed afterwards) and
+`messages: dict[str, ContactMessage]` keyed by a sortable timestamp-random
+id (dict-keyed, so a save's database diff touches only the appended
+message). Each message stores `t`, the Markdown source `md`, its strict
+rendering `html`, `admin` ("" = visitor, else the SSO name) and `page`.
+Appends are single transactions (`contact` / `contact:reply` /
+`contact:read`).
 
 ## Limits and deliberate gaps (MVP)
 
 - Message source ≤ 8000 chars; uploads ≤ 20 MB and ≤ 10 pending per
   session; payloads cap at 100 threads × last 200 messages.
 - No thread moderation (delete/block), no orphan-upload GC for files
-  orphaned by a server restart, panel strings are English-only, and the
-  admin inbox lists threads by token-hash prefix rather than enriched
-  visitor identity.
+  orphaned by a server restart, and panel strings are English-only.