Log keypresses, keep frames flowing when idle, blank the screen when off

Three things reported from actual use, plus the bug the logging exposed.

1. Keys are logged (source "key"), including refusals and presses while powered
   off. Without this there was no way to tell "the key never arrived" from "the
   firmware did something else with it" -- which is exactly what was needed below.

2. /stream resends the current frame every IDLE_FRAME_INTERVAL_S even when nothing
   changed. Change-detection alone made a static screen indistinguishable from a
   dead connection, and a client joining mid-idle stayed blank. Measured: 6 frames
   in 10 idle seconds, where before it was 0.

3. Off now actually blanks the screen. The dark panel moved to a .screenwrap
   wrapper and the <img> is hidden; setting a background on the <img> alone did
   nothing visible, because the image kept painting the last frame over it.

Then the reported bug: in the menu, UP/DOWN behaved like another MENU press. The
key log made it diagnosable and the cause was mine -- LONG_PRESS_AFTER_MS was set
to the firmware's own 400 ms boundary, but a deliberate click runs 100-500 ms, so
ordinary clicks sent tap AND held and the firmware acted on both:

  held DOWN  auto-repeated, gMenuCursor 3 -> 12 from one click
  held MENU  entered the submenu, gIsInSubMenu 0 -> 1

The UI threshold is now 900 ms, well clear of any click, and FIRMWARE_HELD_MS is a
separate constant so the two are not conflated again. Verified against the real
firmware: 120/300/500/800 ms clicks each move the cursor exactly +1 with
submenu=0, while a deliberate 1400 ms hold still auto-repeats (+9).

tools/test_webui.py

@@ -554,5 +554,81 @@ class TestLogPane(unittest.TestCase):
         self.assertIn("clientHeight", self.body)
 
 
+class TestKeyLogging(unittest.TestCase):
+    """Keys must appear in the log, or the UI cannot be debugged from the browser."""
+
+    def test_key_press_is_logged(self):
+        client, sup, http = make_supervised()
+        log = http.application.config["LOG"]
+        http.post("/api/key", json={"key": "MENU", "hold_ms": 60})
+        texts = [e["text"] for e in log.entries() if e["source"] == "key"]
+        self.assertTrue(any("MENU" in t for t in texts), texts)
+
+    def test_log_records_the_hold_duration(self):
+        """Distinguishing a tap from a hold is the whole point of the record."""
+        client, sup, http = make_supervised()
+        log = http.application.config["LOG"]
+        http.post("/api/key", json={"key": "MENU", "hold_ms": 900})
+        texts = [e["text"] for e in log.entries() if e["source"] == "key"]
+        self.assertTrue(any("900" in t for t in texts), texts)
+
+    def test_log_marks_a_long_press_as_held(self):
+        client, sup, http = make_supervised()
+        log = http.application.config["LOG"]
+        http.post("/api/key", json={"key": "MENU", "hold_ms": 900})
+        texts = [e["text"].lower() for e in log.entries() if e["source"] == "key"]
+        self.assertTrue(any("held" in t for t in texts), texts)
+
+    def test_rejected_key_is_logged_too(self):
+        """A refused key must be visible, or it looks like nothing happened."""
+        client, sup, http = make_supervised()
+        log = http.application.config["LOG"]
+        http.post("/api/key", json={"key": "PTT"})
+        texts = [e["text"] for e in log.entries() if e["source"] == "key"]
+        self.assertTrue(any("PTT" in t for t in texts), texts)
+
+
+class TestIdleKeepalive(unittest.TestCase):
+    """A static screen must still produce frames, just slowly."""
+
+    def test_keepalive_interval_is_defined(self):
+        self.assertTrue(hasattr(webui, "IDLE_FRAME_INTERVAL_S"))
+        self.assertGreater(webui.IDLE_FRAME_INTERVAL_S, 0)
+
+    def test_keepalive_is_slower_than_the_live_rate(self):
+        """Slower on idle, but never stopped."""
+        self.assertGreater(webui.IDLE_FRAME_INTERVAL_S, 1.0 / webui.TARGET_FPS)
+
+
+class TestLongPressThresholdIsNotTrippedByNormalClicks(unittest.TestCase):
+    """A deliberate click must not be promoted to a long press.
+
+    Reproduced against the real firmware: optimistic send fires a tap at
+    pointerdown and a held press if the button is still down at the threshold. With
+    the threshold at the firmware's own 400 ms boundary, an ordinary click sent
+    BOTH, and the firmware acted on both --
+      held DOWN  auto-repeated, moving gMenuCursor 3 -> 12 in one click
+      held MENU  entered the submenu (gIsInSubMenu 0 -> 1)
+    which is exactly the reported "UP/DOWN acts like another MENU press".
+    """
+
+    def test_ui_threshold_is_well_above_a_human_click(self):
+        # Deliberate clicks run 100-500 ms. The UI threshold must sit clear of
+        # that range, not at the firmware's 400 ms event boundary.
+        self.assertGreaterEqual(webui.LONG_PRESS_AFTER_MS, 700)
+
+    def test_ui_threshold_is_above_the_firmware_boundary(self):
+        """Still needs to be past 400 ms, or the long press is not 'held'."""
+        self.assertGreater(webui.LONG_PRESS_AFTER_MS, 400)
+
+    def test_long_press_duration_clears_the_firmware_boundary(self):
+        self.assertGreater(webui.LONG_PRESS_MS, 400)
+
+    def test_page_requires_an_intentional_hold(self):
+        _, http = make_app()
+        body = http.get("/").get_data(as_text=True)
+        self.assertIn(f"LONG_PRESS_AFTER_MS = {webui.LONG_PRESS_AFTER_MS}", body)
+
+
 if __name__ == "__main__":
     unittest.main()