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/templates.html

@@ -566,7 +566,16 @@
 			<div class="form-section-label">Upload new PDF</div>
 			<div class="fp-upload-row">
 				<input type="file" id="fp-file-input" accept=".pdf,application/pdf">
-				<input type="text" id="fp-upload-name" placeholder="Display name" autocomplete="off">
+			</div>
+			<div class="fp-upload-row">
+				<label class="fp-field-label">Display name</label>
+				<input type="text" id="fp-upload-name" placeholder="Human readable label" autocomplete="off">
+			</div>
+			<div class="fp-upload-row">
+				<label class="fp-field-label">Filename on disk</label>
+				<input type="text" id="fp-upload-filename" placeholder="e.g. lm741.pdf" autocomplete="off" spellcheck="false">
+			</div>
+			<div class="fp-upload-row">
 				<button type="button" class="btn btn-primary btn-sm" id="fp-upload-btn">Upload</button>
 			</div>
 		</div>