feat(dashboard): pin toggle on tiles + handleDashboardPin handler

Phase 5h slice 4 — adds the star button on each tile that flips
Pinned on the projax item via POST /dashboard/pin.

Backend:
- store.SetPinned(ids, pinned bool) — minimal-write helper that
  mirrors SetPublic, only touching the pinned column.
- web/dashboard_pin.go — handleDashboardPin parses id + pin from
  form, calls SetPinned, invalidates the entire dashboard cache (pin
  affects sort order across every view/scope/filter combo), then
  re-renders by delegating to handleDashboard so HTMX receives the
  updated #dashboard-section HTML.
- Route: POST /dashboard/pin (sibling of /dashboard/task/*).

Frontend:
- Tile template now leads with a <form class="tile-pin-form"> that
  POSTs id + the inverted pin state. Button glyph is ☆ when unpinned,
  ★ when pinned; aria-label flips accordingly.
- HTMX swaps the entire #dashboard-section so the tile moves to the
  pinned-first position (or back to alphabetical) without a full reload.
- CSS: .tile-pin (transparent button, muted color, accent on hover);
  .tile-pin.pinned for the filled-star state.

Test helper: server_test.go gains a post() helper paired with the
existing get() — form-encoded POSTs for writeback tests.

Tests (dashboard_pin_test.go):
- TestDashboardPinTogglesItem — POST pin=true flips the row, and the
  re-render shows the .tile-pinned class on the tile <article>.
- TestDashboardPinUnpinsItem — POST pin=false on a pinned row unpins.
- TestDashboardPinRequiresID — missing id returns 400.
- TestDashboardPinInvalidatesCache — primes with unpinned cache,
  POSTs pin, asserts the next GET reflects the pinned class (proving
  the prior cache entry was busted).

web/static/style.css

@@ -355,7 +355,13 @@ table.bulk .chip-add-btn:hover { background: var(--accent); color: var(--accent-
 .dashboard .tile .tile-head { display: flex; align-items: baseline; gap: 8px; flex-wrap: wrap; }
 .dashboard .tile .tile-title { font-weight: 600; color: var(--fg); text-decoration: none; }
 .dashboard .tile .tile-title:hover { text-decoration: underline; }
-.dashboard .tile .tile-star { color: var(--accent); margin-right: 2px; }
+.dashboard .tile .tile-pin-form { display: inline-flex; margin: 0 4px 0 0; }
+.dashboard .tile .tile-pin {
+  background: transparent; border: none; cursor: pointer;
+  color: var(--muted); padding: 0; font-size: 1.1em; line-height: 1;
+}
+.dashboard .tile .tile-pin.pinned { color: var(--accent); }
+.dashboard .tile .tile-pin:hover { color: var(--accent); }
 .dashboard .tile .tile-path {
   font-family: ui-monospace, SFMono-Regular, monospace;
   font-size: 0.78em;