Strava sync improvements

- Fix first sync finding 0 activities: remove last_sync_at stamp at
    connect time so the first sync checks all Strava history (existence
    check skips already-extracted files without fetching streams)
  - Add POST /api/strava/reset with soft/hard modes: soft sets last_sync_at
    to the most recent activity already on disk; hard clears it entirely
  - Surface error_count in sync response and status message
  - Add Reset / Hard reset buttons below Sync now in the upload modal
  - Reload on bfcache restore so client:only components re-mount after
    back navigation

bincio/edit/server.py

@@ -716,8 +716,6 @@ async def strava_callback(code: str = "", error: str = "") -> RedirectResponse:
         token = exchange_code(strava_client_id, strava_client_secret, code)
     except StravaError:
         return RedirectResponse(f"{site_url}?strava=error")
-    # Stamp last_sync_at at connect time so the first sync only fetches new activities
-    token.setdefault("last_sync_at", int(time.time()))
     save_token(dd, token)
     return RedirectResponse(f"{site_url}?strava=connected")
 
@@ -787,4 +785,50 @@ async def strava_sync() -> JSONResponse:
     token["last_sync_at"] = int(time.time())
     save_token(dd, token)
 
-    return JSONResponse({"ok": True, "imported": imported, "skipped": skipped, "errors": errors[:5]})
+    return JSONResponse({"ok": True, "imported": imported, "skipped": skipped, "error_count": len(errors), "errors": errors[:5]})
+
+
+@app.post("/api/strava/reset")
+async def strava_reset(request: Request) -> JSONResponse:
+    """Reset last_sync_at.
+
+    mode=soft  — set to the started_at of the most recent activity already on disk
+                 (next sync only fetches activities newer than the last known one)
+    mode=hard  — clear last_sync_at entirely
+                 (next sync re-downloads the full Strava history, skipping existing files)
+    """
+    dd = _get_data_dir()
+    from bincio.extract.strava_api import load_token, save_token
+    token = load_token(dd)
+    if token is None:
+        raise HTTPException(400, "Not connected to Strava")
+
+    body = await request.json()
+    mode = body.get("mode", "soft")
+
+    if mode == "hard":
+        token.pop("last_sync_at", None)
+        save_token(dd, token)
+        return JSONResponse({"ok": True, "mode": "hard", "last_sync_at": None})
+
+    # soft: find the most recent started_at in the current index
+    from datetime import datetime, timezone
+    index_path = dd / "index.json"
+    last_ts: int | None = None
+    if index_path.exists():
+        index_data = json.loads(index_path.read_text(encoding="utf-8"))
+        started_ats = [
+            a.get("started_at") for a in index_data.get("activities", [])
+            if a.get("started_at")
+        ]
+        if started_ats:
+            latest = max(started_ats)
+            dt = datetime.fromisoformat(latest.replace("Z", "+00:00"))
+            last_ts = int(dt.astimezone(timezone.utc).timestamp())
+
+    if last_ts is None:
+        token.pop("last_sync_at", None)
+    else:
+        token["last_sync_at"] = last_ts
+    save_token(dd, token)
+    return JSONResponse({"ok": True, "mode": "soft", "last_sync_at": last_ts})