fix(dm): Fix mobile menu link and page height issues

- Change DM menu link from <a> to <button> with onclick for better
  mobile compatibility (fixes link not working on some devices)
- Use identical HTML structure as index.html for proper layout
- Remove conflicting inline styles that caused viewport overflow
- Rely on existing style.css rules for body/main flexbox layout

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/base.html

@@ -63,13 +63,13 @@
                     <i class="bi bi-broadcast-pin" style="font-size: 1.5rem;"></i>
                     <span>Manage Channels</span>
                 </button>
-                <a href="/dm" class="list-group-item list-group-item-action d-flex align-items-center gap-3" data-bs-dismiss="offcanvas">
+                <button class="list-group-item list-group-item-action d-flex align-items-center gap-3" onclick="window.location.href='/dm';">
                     <i class="bi bi-envelope" style="font-size: 1.5rem;"></i>
                     <div class="d-flex flex-grow-1 justify-content-between align-items-center">
                         <span>Direct Messages</span>
                         <span id="dmMenuBadge" class="badge bg-success rounded-pill" style="display: none;">0</span>
                     </div>
-                </a>
+                </button>
                 <div class="list-group-item">
                     <div class="d-flex align-items-center gap-3 mb-2">
                         <i class="bi bi-calendar3" style="font-size: 1.5rem;"></i>