Admin Messages: CSV export of the whole filtered list

A right-aligned 'Export CSV ↓' link in the filter row downloads /_gm/messages.csv
with the active filters (game / sender / name / ext masks), exporting every matching
message (capped at 100k) regardless of the page window — columns time, source,
sender_id, sender, ip, message, game_id.

backend/internal/adminconsole/templates/pages/messages.gohtml

@@ -7,6 +7,7 @@
 <input name="name" value="{{.NameMask}}" placeholder="sender name mask (* ?)">
 <input name="ext" value="{{.ExtMask}}" placeholder="sender external id mask (* ?)">
 <button type="submit">Filter</button>
+<a class="export" href="/_gm/messages.csv?{{.FilterQuery}}">Export CSV ↓</a>
 </form>
 {{if or .GameID .UserID}}
 <p class="note">Filtered{{if .GameID}} to game <a href="/_gm/games/{{.GameID}}">{{.GameID}}</a>{{end}}{{if .UserID}} from <a href="/_gm/users/{{.UserID}}">sender</a>{{end}} · <a href="/_gm/messages">clear</a></p>