feat: Add resend button and adjust message byte limits

- Add Resend button under own messages in channel chat and DM
  (allows easy re-sending of failed messages)
- Change channel chat limit from 140 to 135 bytes
- Change DM limit from 140 to 150 bytes
  (experimentally verified Meshcore limits)
- Remove misleading Hops info from DM message bubbles
- Update README with new byte limits

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

app/static/css/style.css

@@ -526,6 +526,19 @@ main {
     color: #dc3545;
 }
 
+/* DM Action Buttons */
+.dm-actions {
+    display: flex;
+    justify-content: flex-end;
+    margin-top: 0.25rem;
+}
+
+.dm-action-btn {
+    padding: 0.15rem 0.4rem;
+    font-size: 0.75rem;
+    line-height: 1;
+}
+
 /* DM Conversation List Item */
 .dm-conversation-item {
     padding: 0.75rem;