v2 firmware: HA command channel, battery screen, dynamic Home buttons, tilt-dimmer

Watch-side features developed on the AMOLED-2.06 and shared across all boards:

- BLE command channel (…0005, notify watch→PC) via ble_send_command(); the
  watch stays "dumb" and the daemon maps commands to Home Assistant.
- Battery detail screen + protective low-voltage cutoff; power HAL gains
  battery_mv() / shutdown(), with battery_est.{h,cpp} for the time-left anchor.
- Home screen: dynamic 2-column button grid driven by the desktop config
  (RX "btns" labels); tap notifies {"cmd":"btn","i":N}.
- Dimmer screen: IMU tilt-joystick over the light's brightness / color temp.
  imu_hal_read_accel() added to the HAL (real on 2.06/2.16, no-op elsewhere);
  streams absolute {"cmd":"bri"|"ct","v":..}; daemon seeds the dial via "dim".
  Per-board tilt axis is calibrated with the DIM_CALIB overlay (off by default).

Multi-board fix: enable LV_FONT_MONTSERRAT_20/28 on the 2.16, 1.8 and C6 envs.
Shared ui.cpp uses these glyphs for the launcher tiles and back chevron, so
those three boards had failed to compile since the Phase-2 UI landed. All four
envs now build clean. README: correct the Windows pairing name to "Clawdmeter".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/battery_est.cpp

@@ -0,0 +1,55 @@
+#include "battery_est.h"
+#include <Arduino.h>
+
+// Anchor-based, not a ring buffer (cf. usage_rate.cpp): we remember (time, pct)
+// at the moment discharge began and project from the average rate since then.
+//   rate = (anchor_pct - pct_now) / minutes_elapsed
+//   minutes_left = pct_now / rate
+// A long baseline beats a short ring for a signal that only ticks every few
+// minutes — the estimate just keeps tightening as the session runs. We withhold
+// a number until the cell has dropped a meaningful amount over a few minutes, so
+// the first figure isn't built on fuel-gauge settling noise right after boot.
+#define EST_MIN_DROP_PCT     3         // need >= 3% drained since the anchor
+#define EST_MIN_ELAPSED_MS   300000UL  // ...over at least 5 minutes
+#define EST_MAX_MINUTES      (100 * 60)
+
+static bool     have_anchor   = false;
+static uint32_t anchor_ms     = 0;
+static int      anchor_pct    = -1;
+static int      last_pct      = -1;
+static bool     last_charging = false;
+
+void battery_est_update(int percent, bool charging) {
+    last_charging = charging;
+    last_pct      = percent;
+
+    if (charging || percent < 0) {
+        // On USB / charging / no reading the estimate is meaningless. Drop the
+        // anchor so the next discharge starts from a fresh baseline.
+        have_anchor = false;
+        return;
+    }
+    // Discharging. Start an anchor, or re-anchor if the gauge ticked UP (it
+    // relaxes upward when load drops) so we never compute a negative rate.
+    if (!have_anchor || percent > anchor_pct) {
+        have_anchor = true;
+        anchor_ms   = millis();
+        anchor_pct  = percent;
+    }
+}
+
+int battery_est_minutes(void) {
+    if (last_charging || last_pct < 0) return -2;
+    if (!have_anchor)                  return -1;
+
+    int      drop = anchor_pct - last_pct;
+    uint32_t dt   = millis() - anchor_ms;
+    if (drop < EST_MIN_DROP_PCT || dt < EST_MIN_ELAPSED_MS) return -1;
+
+    // Minutes each 1% has been taking, extrapolated across the remaining pct.
+    float minutes_per_pct = (float)dt / 60000.0f / (float)drop;
+    float mins = (float)last_pct * minutes_per_pct;
+    if (mins < 0)              return -1;
+    if (mins > EST_MAX_MINUTES) mins = EST_MAX_MINUTES;
+    return (int)(mins + 0.5f);
+}