Fix #194 r2: align line-number gutter in both wrap modes

QA FAIL of PR #179 (verified on palette-dev 2026-09-10) found two defects:

1) Row accounting: splitLines() joined .codeline blocks with '\n' text
   nodes. Since dee062d made .codeline display:block, each interleaved
   newline text node rendered as its own extra visual row in the code
   column under pre-wrap, but renumber() counted only .codeline span
   heights, so the gutter had one fewer row per line break and numbers
   drifted off alignment on wrapped lines. Fix: join('') so blocks are
   adjacent with no phantom newline rows; blank source lines become
   empty blocks given a line box via .codeline:empty::before (ZWJ) in
   app.css so they still count as one row.

2) Inline gutter spans: .gutline spans were appended with no separators
   and .code .gutter .gutline { display: block } did not exist on the
   deployed build, so all numbers landed on one row. That rule is
   present in origin/dev (merged via #183); kept unchanged and verified
   by rendering in a local build.

Wrap OFF path unchanged: gutter still gets '1\n2\n3\n4' text and the
horizontal-scroll gutter behavior is untouched (no CSS changes to
scrolling; only a :empty line-box rule). CSP unchanged: no inline
styles/handlers, static JS only.

internal/web/static/app.css

@@ -301,6 +301,9 @@ html[data-wrap] .float { overflow-x: hidden; }
 .codeline { display: block; }
 /* #167 rev: gutter number spans must stack one per visual row (wrap on) */
 .code .gutter .gutline { display: block; }
+/* #194: codeline blocks are adjacent (no '\n' text between them), so an
+   empty block (blank source line) needs its own line box to stay one row */
+.codeline:empty::before { content: "\200B"; }
 /* syntax highlight tokens (#1) */
 .tok-kw { color: #c792ea; }
 .tok-str { color: #a5e075; }