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/MessageList.css

@@ -128,6 +128,19 @@
   font-weight: 700;
 }
 
+.message-room-reference {
+  background: color-mix(in srgb, var(--ds-accent) 18%, transparent);
+  color: var(--ds-accent);
+  border-radius: 5px;
+  padding: 0 4px;
+  font-weight: 700;
+  text-decoration: none;
+}
+
+.message-room-reference:hover {
+  text-decoration: underline;
+}
+
 .message-text code {
   background: var(--ds-surface-2);
   padding: 1px 5px;