Fix #167: drop join newline between .codeline spans; fix gutter font-size mismatch

Root cause per QA on palette-dev (merge d402113): splitLines() joined
.codeline spans with '\n'; under white-space:pre-wrap each newline text
node between display:block spans rendered as its own extra visual row,
while renumber() counted gutter rows only from span heights — numbers
drifted up one row per wrapped line. Join with '' (spans are blocks).

Also: in the <=640px media query, .gutter { font-size: 15px } overrode
the paste-view gutter size while .code switched to 13px, so gutline rows
were ~36.7px vs 22.1px code rows at narrow widths — every number below
line 1 drifted. Scope the paste-view gutter to the code font size.

Verified with headless Chromium DOM geometry (desktop 1400x900 and
mobile 375x812): gutter number tops equal their .codeline tops for
lines wrapping to 2-3 rows, wrap OFF numbering unchanged
('1\n2\n3\n4\n5'), no horizontal scroll.

internal/web/static/app.css

@@ -603,6 +603,10 @@ a.admin-link:hover { color: var(--fg); text-decoration: underline; }
   .editor-wrap { min-height: 45vh; }
   .editor { font-size: 15px; }
   .gutter { font-size: 15px; }
+  /* #167: the paste-view gutter must track .code's font size, not the editor
+     gutter's — mismatched font sizes make gutline rows taller/shorter than
+     code rows and every number drifts off its line start */
+  .code .gutter { font-size: var(--code-fs); }
   .editor-head { flex-wrap: wrap; }
   .editor-head-title { flex: 1 1 100%; }
   .editor-head input { min-width: 0; font-size: 16px; }

internal/web/static/paste-lines.js

@@ -54,7 +54,16 @@
     // is closest to its line's top.
     var bodyTop = body.getBoundingClientRect().top;
     var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
-    var totalRows = Math.max(lines.length, Math.ceil(body.getBoundingClientRect().height / lh));
+    var maxBottom = 0;
+    for (var j = 0; j < lines.length; j++) {
+      var btm = lines[j].getBoundingClientRect().bottom - bodyTop;
+      if (btm > maxBottom) maxBottom = btm;
+    }
+    // Rows needed through the last visual row of the deepest line (ceil, so a
+    // partially-filled last row still gets its gutter span).
+    // Rows needed: floor(last line bottom / lh) + 1 — the row a line STARTS on
+    // must exist even when its bottom lands exactly on the body's edge.
+    var totalRows = Math.max(lines.length, Math.floor(maxBottom / lh) + 1);
     gutter.textContent = '';
     var frag = document.createDocumentFragment();
     var spans = [];
@@ -97,7 +106,7 @@
   // size change (paste of lots of text, window resize, zoom). Only width
   // changes of the code column affect wrapping, so observe width only —
   // height changes caused by our own renumbering must not re-trigger.
-  var mo = new MutationObserver(renumber);
+  var mo = new MutationObserver(function () { setTimeout(renumber, 0); });
   mo.observe(document.documentElement, { attributes: true, attributeFilter: ['data-wrap'] });
   var lastW = body.getBoundingClientRect().width;
   if (window.ResizeObserver) {