Escape the .pdf extension in the op-target regex (the unescaped /.pdf$/
matched any path ending in pdf) and fold the copy-pasted toggle button
into PaneToggle; record the sixth deliverable in the design doc.
Records the five features shipped on this PR (md reading mode incl.
mermaid, highlight expansion, secret redaction, html render mode), their
deliberate boundaries (redaction heuristics, saved-file render
semantics), and the 2026-09-06 standards pass.
Menghuan1918