fix(renderer): drop empty trailing line from blockquote match

The new group-based blockquote rule introduced in this PR captures the
trailing newline in its (?:\n|$) clause. After block.split('\n') that
trailing newline produces an empty final element. The original filter
only dropped lone bare '>' artifacts on the last line, so the empty
final element survived, and the .map(blank → '<br>') step turned it
into a phantom <br> immediately before </blockquote>.

Visible symptom: any blockquote whose source ends with \n (the common
case — a quote followed by another paragraph or end-of-message) renders
with an extra blank line at the bottom of the quote.

Reproducer:
  '> Hello\n\nThe rest of the message.'
    → '<blockquote>Hello\n<br></blockquote>\nThe rest of the message.'
                          ^^^ phantom <br>

Fix: replace the single-line filter with a while-loop that pops trailing
lines while they are either empty OR a bare '>'. This matches the
intent the Python test mirror in tests/test_blockquote_rendering.py
already had (the mirror was correct; the JS was not — that's why
the original tests passed despite the bug).

Also add four new regression tests in TestNoPhantomTrailingBr that pin
the no-trailing-<br> invariant for the common shapes:
  - input ending with \n
  - quote followed by paragraph (the real-world case)
  - multi-line quote ending with \n
  - quote with blank continuation + trailing \n (internal <br> stays,
    trailing <br> does not)

Verified end-to-end with node against the actual JS regex.
244 renderer-adjacent tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/ui.js

@@ -769,8 +769,16 @@ function renderMd(raw){
   //   3. After the fenced-code pass, lines of > preceding/following code
   //      blocks were left as literals because .+ didn't match empty lines
   s=s.replace(/((?:^>[^\n]*(?:\n|$))+)/gm,block=>{
-    const inner=block.split('\n')
-      .filter((_,i,a)=>i<a.length-1||_.trim()!='>')  // drop lone trailing '>' artifact
+    const lines=block.split('\n');
+    // Drop trailing artifacts: empty string from a trailing \n in the match
+    // (split adds '' after the final \n) and lone bare '>' lines that
+    // weren't intended as content. Without this, a blockquote whose source
+    // ends with \n (the common case — anything followed by another block)
+    // renders with a phantom <br> before </blockquote>.
+    while(lines.length&&(lines[lines.length-1].trim()===''||lines[lines.length-1].trim()==='>')){
+      lines.pop();
+    }
+    const inner=lines
       .map(l=>l.replace(/^>[ \t]?/,''))               // strip "> " or ">"
       .map(l=>l.trim()===''?'<br>':inlineMd(l))        // blank lines → <br>, text → inlineMd
       .join('\n');

tests/test_blockquote_rendering.py

@@ -162,6 +162,46 @@ class TestInlineMarkdownInsideBlockquote:
         assert "<blockquote>" in out
 
 
+class TestNoPhantomTrailingBr:
+    """The fix must drop both empty trailing lines (from a trailing \\n in the
+    match) and bare '>' artifacts. Without this, the common case — a blockquote
+    followed by another paragraph — renders with a phantom <br> right before
+    </blockquote>, leaving a visible blank line at the bottom of the quote.
+    """
+
+    def test_input_ending_with_newline_no_trailing_br(self):
+        """`> Hello\\n` must NOT produce `<blockquote>Hello\\n<br></blockquote>`."""
+        out = _apply_blockquote("> Hello\n")
+        assert "<br></blockquote>" not in out, (
+            f"Trailing <br> leaked inside the blockquote (phantom blank line): {out!r}"
+        )
+
+    def test_blockquote_followed_by_paragraph_no_trailing_br(self):
+        """The common real-world shape: quote + blank line + paragraph."""
+        src = "> Quoted text\n\nNormal paragraph"
+        out = _apply_blockquote(src)
+        assert "<br></blockquote>" not in out, (
+            f"Trailing <br> leaked inside blockquote when followed by paragraph: {out!r}"
+        )
+
+    def test_multiline_quote_ending_with_newline_no_trailing_br(self):
+        out = _apply_blockquote("> Line one\n> Line two\n")
+        assert "<br></blockquote>" not in out, (
+            f"Multi-line quote ending with \\n must not leave a trailing <br>: {out!r}"
+        )
+
+    def test_quote_with_blank_continuation_then_newline(self):
+        """`> A\\n>\\n> B\\n` — the internal `<br>` for the blank line stays,
+        but the trailing newline must not add a second `<br>` at the end."""
+        out = _apply_blockquote("> A\n>\n> B\n")
+        # Internal <br> for the blank-line continuation is intentional
+        assert "<br>" in out
+        # But there must not be a <br> immediately before the closing tag
+        assert "<br></blockquote>" not in out, (
+            f"Trailing <br> leaked at end of blockquote: {out!r}"
+        )
+
+
 class TestBlockquoteFollowedByParagraph:
     """A blockquote followed by a normal paragraph must not bleed into each other."""