PDF: separate display name and filename; show filename in picker; fix rename

- Upload dialog now has distinct display name + filename fields, both pre-filled
  from the uploaded file but independently editable
- Rename in file picker shows and edits both display name and filename separately
- Filename conflict checked against both KV store and disk (via rename_no_replace)
- Display name and filename are fully independent — no longer derived from each other
- Add find_pdf_references() helper in storage.mjs for future use
- CSS: fp-name-wrap shows display name + dim monospace filename below it;
  rename mode stacks two inputs; fp-field-label for upload form labels

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/storage.mjs

@@ -169,6 +169,11 @@ export function delete_pdf(id) {
 	return store.delete(`pdf:${id}`);
 }
 
+// Returns all components that reference the given PDF id in their file_ids array.
+export function find_pdf_references(pdf_id) {
+	return list_components().filter(c => c.file_ids?.includes(pdf_id));
+}
+
 // --- Grid images ---
 
 export function list_grid_images() {