feat(composer): a single pasted/dropped image lands in its caption field (#129)

Verified first: after a paste the focus stayed in the composer and the
caption needed a click. Now, when exactly one image is pasted or dropped
into an empty composer, its upload card's caption input takes focus;
Enter there sends the board (with any composer text) and Escape returns
focus to the composer. Multi-file drops and non-empty composers are
unchanged. The target card is matched by file name + mtime because the
metadata strip re-wraps the File.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/upload-card/UploadCardRenderer.tsx

@@ -283,6 +283,12 @@ type UploadCardRendererProps = {
   setMetadata: (fileItem: TUploadItem, metadata: TUploadMetadata) => void;
   onRemove: (file: TUploadContent) => void;
   onComplete?: (upload: UploadSuccess) => void;
+  /** [Gitea #129] Focus the caption field on mount (single image into an empty composer). */
+  autoFocusCaption?: boolean;
+  /** Enter in the caption field: send the board (and any composer text). */
+  onCaptionSubmit?: () => void;
+  /** Escape in the caption field: hand focus back to the composer. */
+  onCaptionEscape?: () => void;
 };
 export function UploadCardRenderer({
   isEncrypted,
@@ -290,6 +296,9 @@ export function UploadCardRenderer({
   setMetadata,
   onRemove,
   onComplete,
+  autoFocusCaption,
+  onCaptionSubmit,
+  onCaptionEscape,
 }: UploadCardRendererProps) {
   const mx = useMatrixClient();
   const mediaConfig = useMediaConfig();
@@ -379,6 +388,16 @@ export function UploadCardRenderer({
               size="300"
               radii="300"
               style={{ marginTop: config.space.S200, width: '100%' }}
+              autoFocus={autoFocusCaption}
+              onKeyDown={(e: React.KeyboardEvent<HTMLInputElement>) => {
+                if (e.key === 'Enter' && !e.shiftKey && !e.nativeEvent.isComposing) {
+                  e.preventDefault();
+                  onCaptionSubmit?.();
+                } else if (e.key === 'Escape') {
+                  e.preventDefault();
+                  onCaptionEscape?.();
+                }
+              }}
             />
           )}
           <CompressionCheckbox fileItem={fileItem} metadata={metadata} setMetadata={setMetadata} />