Add session-break detection and clearer recording instructions

src/phyphox_app_block.py

@@ -75,8 +75,9 @@ def render_phyphox_tab(
 1. Install [Phyphox](https://phyphox.org/) on your phone
 2. Open **Acceleration (with g)** and **Gyroscope**: record simultaneously
 3. Hold the phone at your waist (same position as the UCI dataset)
-4. Record at least 3 seconds of a single activity
-5. Export both experiments as CSV and upload below
+4. **Before recording:** tap the trash icon in Phyphox to clear any previous data
+5. Record 10–15 seconds of a single activity without pausing
+6. Export both experiments as CSV and upload below
     """)
 
     if "phyphox_run" not in st.session_state:
@@ -125,6 +126,21 @@ def render_phyphox_tab(
             return
         else:
             st.caption(f"Signal check: mean magnitude = {_mag:.2f} m/s² (gravity present)")
+
+        # Detect Phyphox pause/resume sessions: large gaps in the time column
+        # mean different activities were stitched together in one CSV.
+        _t = pd.to_numeric(_df["Time (s)"], errors="coerce").dropna().values
+        _gaps = np.diff(_t)
+        _expected_dt = np.median(_gaps)
+        _session_breaks = int(np.sum(_gaps > _expected_dt * 20))
+        if _session_breaks > 0:
+            st.warning(
+                f"**{_session_breaks} session break(s) detected** in this recording. "
+                "Phyphox accumulates data across pause/resume cycles — your CSV contains "
+                f"{_session_breaks + 1} separate recordings stitched together. "
+                "Only windows from a single activity will predict correctly. "
+                "To fix: tap the **trash icon** in Phyphox to clear data before each new recording."
+            )
     except Exception:
         pass