feat(web): mobile image upload to active session via paste dialog (#101)

- Extend the keyboard accessory paste dialog with an image picker
  (camera / photo library) plus best-effort image paste, routing
  selected files through the existing _uploadAndInsertImages pipeline
- Re-encode images to standard JPEG/PNG in the browser before upload,
  so mislabeled gallery images (e.g. MIUI WebP claiming image/jpeg)
  pass the server magic-byte check; PNG keeps transparency, GIF passes
  through untouched, decode failures fall back to the original file
- Log the real byte header on the paste-image magic-mismatch branch to
  pin down any remaining format mismatches without a guessing loop
- Ignore the runtime .claude-images/ upload directory

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>

src/web/public/mobile.css

@@ -1043,7 +1043,7 @@ html.mobile-init .file-browser-panel {
     margin-top: 10px;
   }
 
-  .paste-cancel, .paste-new, .paste-send {
+  .paste-cancel, .paste-new, .paste-send, .paste-image {
     padding: 8px 18px;
     border: none;
     border-radius: 8px;
@@ -1051,6 +1051,14 @@ html.mobile-init .file-browser-panel {
     cursor: pointer;
   }
 
+  /* Image attach button — left-aligned, accent outline */
+  .paste-image {
+    margin-right: auto;
+    background: var(--bg-tertiary, #333);
+    color: var(--accent-color, #7aa2f7);
+    border: 1px solid var(--accent-color, #7aa2f7);
+  }
+
   .paste-cancel {
     background: var(--bg-tertiary, #333);
     color: var(--text-secondary, #aaa);