feat(viewer): MSK datetime + done-cutoff expander [v0.3.0]

Closes 2 more UX-round1 fixes (5/6 done; task-numbers remains
design-first cross-repo, awaiting user decision).

ux-full-datetime: formatMskDateTime(iso) → "YYYY-MM-DD HH:MM МСК".
UTC+3 fixed (no-DST since 2011). Tooltip retains full ISO for
copy-paste. Relative age stays in card-meta. Mirrored in board.js
renderCard re-render path.

ux-done-cutoff: source unchanged (render-side cutoff). Done column
sorts by last_commit_iso desc, first 10 visible, rest get
data-overflow="true" (CSS hides). Column bottom gets
"show N more" expander; click toggles col.show-overflow class +
button text "collapse". Non-done columns untouched.

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

static/board.css

@@ -210,6 +210,33 @@ body.show-archived .card[data-archived="true"] {
   display: block;
 }
 
+.card[data-overflow="true"] {
+  display: none;
+}
+
+.col.show-overflow .card[data-overflow="true"] {
+  display: block;
+  opacity: 0.7;
+}
+
+.done-expander {
+  align-self: flex-start;
+  margin-top: 0.4rem;
+  font: inherit;
+  font-size: 0.75rem;
+  padding: 0.2rem 0.6rem;
+  background: transparent;
+  border: 1px dashed var(--border);
+  border-radius: var(--radius);
+  color: var(--muted);
+  cursor: pointer;
+}
+
+.done-expander:hover {
+  border-style: solid;
+  border-color: var(--muted);
+}
+
 .card[data-hidden="true"] {
   display: none;
 }