Fix Strava re-auth when credentials change; add disconnect button

When a user saves new Strava credentials with a different client_id,
auto-delete the existing token (it belongs to a different OAuth app
and will always fail on refresh). Add POST /api/strava/disconnect
endpoint and a "Disconnect from Strava" button in settings, visible
only when connected.

Immediate: deleted diego_p's stale token so he can reconnect.

bincio/serve/server.py

@@ -1760,6 +1760,18 @@ async def me_set_strava_credentials(
                 pass
     if not csec:
         raise HTTPException(400, "client_secret is required (no existing secret to preserve)")
+
+    # If the client_id changed, the existing token belongs to a different OAuth
+    # app and will fail on refresh — delete it so the user must re-authenticate.
+    token_path = _get_data_dir() / user.handle / "strava_token.json"
+    if creds_path.exists() and token_path.exists():
+        try:
+            old_cid = str(json.loads(creds_path.read_text(encoding="utf-8")).get("client_id", "")).strip()
+            if old_cid and old_cid != cid:
+                token_path.unlink(missing_ok=True)
+        except Exception:
+            pass
+
     creds_path.write_text(
         json.dumps({"client_id": cid, "client_secret": csec}, indent=2),
         encoding="utf-8",
@@ -2470,6 +2482,15 @@ async def strava_status(bincio_session: Optional[str] = Cookie(default=None)) ->
     })
 
 
+@app.post("/api/strava/disconnect")
+async def strava_disconnect(bincio_session: Optional[str] = Cookie(default=None)) -> JSONResponse:
+    """Remove the stored Strava token, forcing a fresh OAuth on next connect."""
+    user = _require_user(bincio_session)
+    token_path = _get_data_dir() / user.handle / "strava_token.json"
+    token_path.unlink(missing_ok=True)
+    return JSONResponse({"ok": True})
+
+
 @app.post("/api/strava/reset")
 async def strava_reset(request: Request, bincio_session: Optional[str] = Cookie(default=None)) -> JSONResponse:
     """Reset last_sync_at so the next sync re-fetches from a chosen point.