phase: 134_sync_cancel

**Phase 134 final verification pass — all green** (all 3 task files already in `complete/`; no implementation was missing)

- Verified: 5-state `SyncStatus` + `cancelled` terminal (error None, final counts kept), `POST /api/sync/cancel` (202/409/403), cooperative `_check_cancelled` boundaries, shutdown `CancelledError` contract preserved, `#sync-cancel-btn` show/hide + neutral "Sync cancelled" settle, 8-branch poll tree, re-attach — code, unit, integration, and E2E layers all present
- `uv run pytest --cov=app --cov-report=term-missing` → **3150 passed**, `app/` coverage **99%** (gate: >90%)
- `uv run pytest tests/e2e/test_sync_cancel.py -v --no-cov` (isolated) → **3 passed** (47.6 s)
- `uv run ruff check .` → clean; `uv run pyright` → 0 errors, 0 warnings

**Completion criteria:** ① cancel endpoint 202/409/403 ✅ (unit + E2E) ② `cancelled` terminal reported by status, error null, counts kept ✅ (integration `test_cancel_mid_run_lands_in_cancelled_then_resyncs_clean`) ③ unrequested-cancel pin ✅ (`test_unrequested_cancel_is_never_swallowed_and_never_converts`) ④ UI lifecycle + clean re-sync (sentinel row, full 40-file import) ✅ (E2E) ⑤ suite/coverage/E2E gates ✅ ⑥ uncancelled runs byte-identical ✅ (all 3150 pre-existing tests green untouched)

- Docs: README.md reviewed in full against code (pages, auth endpoints, every config default vs `app/config.py`, palette 9+8, fail-loud, cancel section) — consistent, no changes needed. AGENTS.md: fixed one pre-existing inaccuracy (rule 2 "move its file" → "move its directory" — phases are directories); everything else verified accurate
- No live infrastructure touched; no deviations

**Next pending phase:** `135_git_source_branch`

tests/unit/test_frontend_sync_upload.py

@@ -82,6 +82,16 @@ contract as the count part, and the UNTRUNCATED full label — main +
 --mono, the rule right after .sync-label-count). A node harness (the
 test-document-viewer pattern) behaviorally pins the rendered parts +
 the full title across every label state.
+
+Phase 134 (task 02) extends the node harness to the Cancel control:
+the extracted functions gain the REAL show/hide helpers + the REAL
+settleSyncButton (step 8 now runs the settle itself — it goes through
+the writer as before, plus it hides the Cancel control), and every
+row snapshots the control's hidden/disabled pair — the behavioral
+contract: shown + enabled in EVERY sync-running state (the entry
+shows it for kind "sync"), hidden + disabled for the upload run
+(the upload job is NOT cancellable in this phase) and after the
+settle (settleSyncButton always hides it).
 """
 from __future__ import annotations
 
@@ -514,6 +524,16 @@ const syncBtn = {
   classList: { add() {}, remove() {} },
 };
 const syncIcon = { classList: { add() {}, remove() {} } };
+// Phase 134 (task 02): the Cancel control — ships hidden + disabled
+// (the markup's state); the extracted REAL show/hide helpers mutate
+// it (show also sets the aria-label — the attrs pocket records it).
+const syncCancelBtn = {
+  hidden: true,
+  disabled: true,
+  attrs: {},
+  setAttribute(k, v) { this.attrs[k] = v; },
+  removeAttribute(k) { delete this.attrs[k]; },
+};
 """
 
 _CHUNK_NODE_TAIL = """
@@ -523,6 +543,7 @@ const show = (tag) => {
     syncLabelMain.textContent,
     syncLabelCount.textContent, syncLabelCount.hidden,
     syncLabelChunk.textContent, syncLabelChunk.hidden,
+    syncCancelBtn.hidden, syncCancelBtn.disabled,
   ]));
 };
 // 1. import, batch in flight (the task's fixture values, 14/38):
@@ -561,8 +582,10 @@ enterSyncRunningState("sync", null, 4, 4, {
   summaries_done: 2, summaries_total: 3, current_chunk: 0, chunks_total: 0,
 });
 show("summaries");
-// 8. the idle settle goes through the writer with no chunk text:
-setSyncLabel("Sync sources");
+// 8. the idle settle goes through the REAL settle (phase 134 task
+//    02: it hides the Cancel control too) — the settle itself goes
+//    through the writer with no chunk text:
+settleSyncButton("Sync sources");
 show("idle");
 """
 
@@ -590,16 +613,34 @@ test-document-viewer pattern). The pinned "· chunk n/m" part (the
       status), the upload run (no status, D2), overview, summaries (its
       own (n/m) is its position), the idle settle — leaves the chunk
       part hidden + empty.
+
+    Phase 134 (task 02): every row also snapshots the Cancel
+    control's hidden/disabled pair — the behavioral contract:
+    shown + enabled in EVERY sync-running state, hidden + disabled
+    for the upload run (the upload job is NOT cancellable in this
+    phase) and after the (real) settle.
     """
     js = _js()
     fns = "\n".join(
         _fn(js, name)
-        for name in ("fmtSyncLabel", "fmtSyncPhaseLabel", "setSyncLabel", "enterSyncRunningState")
+        for name in (
+            "fmtSyncLabel",
+            "fmtSyncPhaseLabel",
+            "setSyncLabel",
+            # Phase 134 (task 02): the running entry's REAL cancel
+            # show/hide + the REAL settle (step 8) — extracted against
+            # the syncCancelBtn stub.
+            "showSyncCancelBtn",
+            "hideSyncCancelBtn",
+            "enterSyncRunningState",
+            "settleSyncButton",
+        )
     )
     script = _CHUNK_NODE_HEAD + fns + _CHUNK_NODE_TAIL
     rows = [json.loads(line) for line in _run_node(script).splitlines() if line]
     by_tag = {row[0]: row for row in rows}
-    # row = [tag, title, announcer, main, count, countHidden, chunk, chunkHidden]
+    # row = [tag, title, announcer, main, count, countHidden, chunk,
+    #        chunkHidden, cancelHidden, cancelDisabled]
 
     # 1. the in-flight import — the chunk part renders, the full
     #    untruncated label rides the title + the announcer (A4).
@@ -634,6 +675,24 @@ test-document-viewer pattern). The pinned "· chunk n/m" part (the
         assert row[2] == row[1], (tag, row)
         assert row[6] == "" and row[7] is True, f"{tag} must hide the chunk part"
 
+    # Phase 134 (task 02): the Cancel control's behavioral contract —
+    # shown + enabled in EVERY sync-running state (the entry shows it
+    # for kind "sync"), hidden + disabled for the upload run (the
+    # upload job is NOT cancellable in this phase) and hidden +
+    # disabled after the real settle (settleSyncButton always hides
+    # it — every terminal + idle passes through it).
+    for tag in ("import-inflight", "import-prebatch", "prelude", "click", "overview", "summaries"):
+        row = by_tag[tag]
+        assert row[8] is False and row[9] is False, (
+            f"{tag}: the Cancel control must be shown + enabled while a sync run is in flight"
+        )
+    for tag in ("upload", "idle"):
+        row = by_tag[tag]
+        assert row[8] is True and row[9] is True, (
+            f"{tag}: the Cancel control must be hidden + disabled "
+            "(the upload job is not cancellable / the settle hides it)"
+        )
+
 
 def test_every_other_state_passes_no_chunk_text() -> None:
     """Source-level pin of the render-only-while-chunks_total>0