Migrate message forwarding to backend REST endpoint

Both real mobile clients (iOS, Android) forward messages via POST
/v1/im/messages/forward, never a client-side {pub} packet -- confirmed
by reading their source directly. Switches web's single-message and
bulk forward paths to the same endpoint, which copies message content
verbatim regardless of shape, eliminating the client-side flat-content
detection this used to require. Comment-sending stays on the existing
sendText() path, unchanged, to preserve current ordering.

Removes send.ts's now-unused forwardMessage/flatForwardPayload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/features/messages/HistorySearchModal.tsx

@@ -78,8 +78,9 @@ function MediaThumb({ item, isVideo }: { item: MediaItem; isVideo: boolean }) {
  *  being modeled on `SearchHit`, same as favorites.ts's own snapshot). Handing that straight to
  *  `renderDrafty` used to crash the WHOLE APP the moment a keyword matched one (`Drafty.toPlainText`
  *  returns `undefined` for a shape with no `.txt`, and `highlightMatches` then called `.toLowerCase()`
- *  on it -- this app has no error boundary anywhere to contain that). Same root cause, and same fix
- *  approach, as `forwardMessage()`'s own `flatForwardPayload` (lib/tinode/send.ts). */
+ *  on it -- this app has no error boundary anywhere to contain that). Same flat-vs-Drafty shape
+ *  distinction `dispatch.ts`'s own `mobileAttachmentContentOf`/`contactCardContentOf` already
+ *  document for message bubbles. */
 function SnapshotContent({ item, highlightQuery }: { item: SearchHit; highlightQuery?: string }) {
   const content = item.snapshot?.content;
   if (!content) return <span className="text-text-tertiary-4">（内容不可用）</span>;