Add #roomname references in chat messages (#47)

Mirrors the existing @-mention system's shape: a regex finds #roomname
tokens, extract_referenced_room_ids validates them against rooms the
*sender* actually belongs to (mirrors mentions' "must be a real member"
rule -- referencing a private room the sender isn't in would otherwise
leak its existence), and a MessageRoomReference join row is stored per
match in create_message. No notification/unread layer, unlike mentions --
referencing a room has no "you were referenced" semantics.

Rendering is the same markdown-link rewrite trick MessageContent.tsx
already uses for mentions (#username -> [#username](mention:username)),
but resolved against the *viewer's* own room list (threaded down from
ChatShellPage's room state through ChatPane/MessageList) rather than the
stored server-side reference -- a reference to a room the current viewer
isn't in quietly renders as plain text instead of a link, same as an
@mention of someone outside the room does. The href scheme renders a
real react-router Link instead of mentions' inert span, since a room
reference is meant to be navigable.

mention_service.strip_code_spans (was _strip_code_spans) is now shared
between both extraction paths rather than private to one module.

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

frontend/src/components/ChatPane.tsx

@@ -9,6 +9,7 @@ import './ChatPane.css'
 
 interface ChatPaneProps {
   room: MyRoomItem
+  rooms: MyRoomItem[]
   members: RoomMember[]
   isMobile: boolean
   onBack: () => void
@@ -20,6 +21,7 @@ interface ChatPaneProps {
 
 export function ChatPane({
   room,
+  rooms,
   members,
   isMobile,
   onBack,
@@ -192,6 +194,11 @@ export function ChatPane({
     [history, live],
   )
 
+  // #47: name -> id for every room this user belongs to, so #roomname
+  // references can resolve to a real link -- deliberately the viewer's own
+  // rooms, not the sender's (see MessageContent.tsx's myRooms prop comment).
+  const myRooms = useMemo(() => new Map(rooms.map((r) => [r.name, r.id])), [rooms])
+
   const connected = socket.connected
   const send = useCallback(
     (content: string, imageId?: string, fileId?: string) => socket.send(room.id, content, imageId, fileId),
@@ -246,6 +253,7 @@ export function ChatPane({
         roomId={room.id}
         messages={messages}
         members={members}
+        myRooms={myRooms}
         onEdit={sendEdit}
         onReact={sendReaction}
       />