refactor(dm): Replace modal with full-page view and fix sent DM tracking

- Replace DM modal with full-page view at /dm route for better mobile UX
- Add workaround for meshcore-cli bug where SENT_MSG contains sender's
  name instead of recipient - now saving sent DMs to separate log file
- Fix DM button styling to match Reply button (btn-outline-secondary)
- Add dm.js for DM page functionality
- Add dm.html template with green navbar for visual distinction
- Update menu link to navigate to /dm instead of opening modal
- Remove unused DM modal functions from app.js
- Update documentation with new DM workflow

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

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

app/static/css/style.css

@@ -445,12 +445,7 @@ main {
     max-width: 100%;
 }
 
-/* DM Button on channel messages */
-.btn-dm {
-    font-size: 0.65rem;
-    padding: 0.1rem 0.3rem;
-    margin-left: 0.25rem;
-}
+/* DM Button on channel messages - uses same class as Reply (.btn-reply) */
 
 /* DM Empty State */
 .dm-empty-state {