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/style.css

@@ -1701,22 +1701,44 @@ nav {
 	background: var(--surface-raised);
 }
 
-.fp-name {
+.fp-name-wrap {
 	flex: 1;
+	display: flex;
+	flex-direction: column;
+	gap: 0.1rem;
+	overflow: hidden;
+	min-width: 0;
+}
+
+.fp-name {
 	font-size: 0.9rem;
 	overflow: hidden;
 	text-overflow: ellipsis;
 	white-space: nowrap;
 }
 
+.fp-filename {
+	font-size: 0.75rem;
+	color: var(--text-dim);
+	font-family: var(--font-mono);
+	overflow: hidden;
+	text-overflow: ellipsis;
+	white-space: nowrap;
+}
+
 .fp-rename-input {
-	flex: 1;
 	font-size: 0.9rem;
 	background: var(--bg);
 	color: var(--text);
 	border: 1px solid var(--border-focus);
 	border-radius: 4px;
 	padding: 0.2rem 0.4rem;
+	width: 100%;
+}
+
+.fp-rename-filename {
+	font-family: var(--font-mono);
+	font-size: 0.8rem;
 }
 
 .fp-upload-section {
@@ -1729,7 +1751,14 @@ nav {
 	display: flex;
 	gap: 0.5rem;
 	align-items: center;
-	flex-wrap: wrap;
+	margin-top: 0.4rem;
+}
+
+.fp-field-label {
+	font-size: 0.85rem;
+	color: var(--text-dim);
+	white-space: nowrap;
+	min-width: 8rem;
 }
 
 .fp-upload-row input[type=text] {