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>

public/lib/api.mjs

@@ -46,13 +46,14 @@ export const delete_component_template  = (id) => req('DELETE', `/api/component-
 
 // PDF files
 export const get_pdfs    = () => req('GET', '/api/pdfs');
-export const rename_pdf  = (id, display_name) => req('PUT', `/api/pdfs/${id}`, { display_name });
+export const rename_pdf  = (id, display_name, filename) => req('PUT', `/api/pdfs/${id}`, { display_name, filename });
 export const delete_pdf  = (id) => req('DELETE', `/api/pdfs/${id}`);
 
-export async function upload_pdf(file, display_name) {
+export async function upload_pdf(file, display_name, filename) {
 	const form = new FormData();
 	form.append('file', file);
 	if (display_name) form.append('display_name', display_name);
+	if (filename) form.append('filename', filename);
 	const res = await fetch('/api/pdfs', { method: 'POST', body: form });
 	const data = await res.json();
 	if (!data.ok) throw new Error(data.error ?? 'Upload failed');