feat: part lifespan tracking in gear tab

API (gear.py):
- POST   /api/gear/{id}/parts
- PATCH  /api/gear/{id}/parts/{pid}
- DELETE /api/gear/{id}/parts/{pid}
- POST   /api/gear/{id}/parts/{pid}/replacements
- DELETE /api/gear/{id}/parts/{pid}/replacements/{rid}

UI (AthleteView.svelte):
- Gear rows are now accordion-expandable
- Collapsed row shows colored status dots (green/yellow/red) per part
- Expanded section: parts list with km-since-replacement colored by threshold,
  Replaced button with date+note form, recent log entries, add-part form
- Contextual suggestion for first part (chain for bikes, shoes for running)
- Edit/delete gear moved into expanded section

bincio/serve/routers/gear.py

@@ -132,3 +132,161 @@ async def gear_delete(
 
     _save(user_dir, items)
     return JSONResponse({"ok": True})
+
+
+# ── Parts ─────────────────────────────────────────────────────────────────────
+
+def _find_item(items: list[dict], item_id: str) -> tuple[int, dict]:
+    idx = next((i for i, g in enumerate(items) if g["id"] == item_id), None)
+    if idx is None:
+        raise HTTPException(404, "Gear item not found")
+    return idx, items[idx]
+
+
+@router.post("/api/gear/{item_id}/parts")
+async def part_add(
+    item_id: str,
+    request: Request,
+    bincio_session: str | None = Cookie(default=None),
+) -> JSONResponse:
+    user = deps._require_user(bincio_session)
+    user_dir = deps._get_data_dir() / user.handle
+    items = _load(user_dir)
+    idx, item = _find_item(items, item_id)
+
+    body = await request.json()
+    name = str(body.get("name", "")).strip()
+    if not name:
+        raise HTTPException(400, "name is required")
+    threshold_km = body.get("threshold_km")
+    if threshold_km is not None:
+        try:
+            threshold_km = float(threshold_km)
+        except (TypeError, ValueError):
+            raise HTTPException(400, "threshold_km must be a number")
+
+    part: dict = {"id": str(uuid.uuid4()), "name": name, "replacements": []}
+    if threshold_km is not None:
+        part["threshold_km"] = threshold_km
+
+    item = dict(item)
+    item.setdefault("parts", [])
+    item["parts"] = [*item["parts"], part]
+    items[idx] = item
+    _save(user_dir, items)
+    return JSONResponse({"ok": True, "part": part}, status_code=201)
+
+
+@router.patch("/api/gear/{item_id}/parts/{part_id}")
+async def part_update(
+    item_id: str,
+    part_id: str,
+    request: Request,
+    bincio_session: str | None = Cookie(default=None),
+) -> JSONResponse:
+    user = deps._require_user(bincio_session)
+    user_dir = deps._get_data_dir() / user.handle
+    items = _load(user_dir)
+    idx, item = _find_item(items, item_id)
+
+    parts = list(item.get("parts", []))
+    pidx = next((i for i, p in enumerate(parts) if p["id"] == part_id), None)
+    if pidx is None:
+        raise HTTPException(404, "Part not found")
+
+    body = await request.json()
+    part = dict(parts[pidx])
+    if "name" in body:
+        name = str(body["name"]).strip()
+        if not name:
+            raise HTTPException(400, "name cannot be empty")
+        part["name"] = name
+    if "threshold_km" in body:
+        try:
+            part["threshold_km"] = float(body["threshold_km"])
+        except (TypeError, ValueError):
+            raise HTTPException(400, "threshold_km must be a number")
+
+    parts[pidx] = part
+    item = {**item, "parts": parts}
+    items[idx] = item
+    _save(user_dir, items)
+    return JSONResponse({"ok": True, "part": part})
+
+
+@router.delete("/api/gear/{item_id}/parts/{part_id}")
+async def part_delete(
+    item_id: str,
+    part_id: str,
+    bincio_session: str | None = Cookie(default=None),
+) -> JSONResponse:
+    user = deps._require_user(bincio_session)
+    user_dir = deps._get_data_dir() / user.handle
+    items = _load(user_dir)
+    idx, item = _find_item(items, item_id)
+
+    parts = [p for p in item.get("parts", []) if p["id"] != part_id]
+    items[idx] = {**item, "parts": parts}
+    _save(user_dir, items)
+    return JSONResponse({"ok": True})
+
+
+@router.post("/api/gear/{item_id}/parts/{part_id}/replacements")
+async def replacement_add(
+    item_id: str,
+    part_id: str,
+    request: Request,
+    bincio_session: str | None = Cookie(default=None),
+) -> JSONResponse:
+    """Log a replacement event for a part. date defaults to today (UTC)."""
+    from datetime import UTC, datetime
+
+    user = deps._require_user(bincio_session)
+    user_dir = deps._get_data_dir() / user.handle
+    items = _load(user_dir)
+    idx, item = _find_item(items, item_id)
+
+    parts = list(item.get("parts", []))
+    pidx = next((i for i, p in enumerate(parts) if p["id"] == part_id), None)
+    if pidx is None:
+        raise HTTPException(404, "Part not found")
+
+    body = await request.json()
+    date = str(body.get("date", "")).strip() or datetime.now(UTC).strftime("%Y-%m-%d")
+    note = str(body.get("note", "")).strip() or None
+
+    entry: dict = {"id": str(uuid.uuid4()), "date": date}
+    if note:
+        entry["note"] = note
+
+    part = dict(parts[pidx])
+    part["replacements"] = [*part.get("replacements", []), entry]
+    parts[pidx] = part
+    items[idx] = {**item, "parts": parts}
+    _save(user_dir, items)
+    return JSONResponse({"ok": True, "replacement": entry}, status_code=201)
+
+
+@router.delete("/api/gear/{item_id}/parts/{part_id}/replacements/{replacement_id}")
+async def replacement_delete(
+    item_id: str,
+    part_id: str,
+    replacement_id: str,
+    bincio_session: str | None = Cookie(default=None),
+) -> JSONResponse:
+    user = deps._require_user(bincio_session)
+    user_dir = deps._get_data_dir() / user.handle
+    items = _load(user_dir)
+    idx, item = _find_item(items, item_id)
+
+    parts = list(item.get("parts", []))
+    pidx = next((i for i, p in enumerate(parts) if p["id"] == part_id), None)
+    if pidx is None:
+        raise HTTPException(404, "Part not found")
+
+    part = dict(parts[pidx])
+    part["replacements"] = [r for r in part.get("replacements", []) if r["id"] != replacement_id]
+    parts[pidx] = part
+    items[idx] = {**item, "parts": parts}
+    _save(user_dir, items)
+    return JSONResponse({"ok": True})