Bot: global keyboard in BotScreen, placeholder expansion via MsgExpand.h

- Replace BotScreen's custom embedded keyboard with the shared global
  keyboard layout (KB_* constants): char rows at KB_CHARS_Y, special row
  at KB_SPECIAL_Y=48 with 5 buttons ([^][Sp][Del][{}][OK]), fixing the
  previous layout that rendered below the 64 px display boundary.
  Proportional column scaling between char rows (10 cols) and special row
  (5 cols). [{}] opens the placeholder picker overlay.

- Add MsgExpand.h: standalone inline expandMsg() with no framework
  dependencies, expanding {loc} and {time} placeholders. Used by both
  MyMesh (bot channel/DM replies) and QuickMsgScreen, replacing the
  duplicate expandBotMsg() static and the inline expandMsg() method.

- Bump FIRMWARE_VERSION to v1.15.3.

- ToolsScreen: remove key-hint footer line.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/MsgExpand.h

@@ -0,0 +1,42 @@
+#pragma once
+#include <cstring>
+#include <cstdio>
+#include <ctime>
+#include <cstdint>
+
+// Expands {loc} and {time} placeholders in tmpl into out (out_len bytes).
+//   lat/lon   — GPS coordinates; gps_valid must be true for {loc} to show coords
+//   utc_ts    — unix timestamp from RTC (0 or <1e9 = no valid time)
+//   tz_hours  — local timezone offset from UTC (-12..+14)
+inline void expandMsg(const char* tmpl, char* out, int out_len,
+                      double lat, double lon, bool gps_valid,
+                      uint32_t utc_ts, int8_t tz_hours) {
+  int oi = 0;
+  const char* p = tmpl;
+  while (*p && oi < out_len - 1) {
+    if (strncmp(p, "{loc}", 5) == 0) {
+      char lb[32];
+      if (gps_valid)
+        snprintf(lb, sizeof(lb), "%.5f,%.5f", lat, lon);
+      else
+        strcpy(lb, "no GPS");
+      int ll = strlen(lb);
+      if (oi + ll < out_len - 1) { memcpy(out + oi, lb, ll); oi += ll; }
+      p += 5;
+    } else if (strncmp(p, "{time}", 6) == 0) {
+      if (utc_ts > 1000000000UL) {
+        uint32_t local_ts = utc_ts + (int32_t)tz_hours * 3600;
+        time_t t = (time_t)local_ts;
+        struct tm* ti = gmtime(&t);
+        char tb[8];
+        snprintf(tb, sizeof(tb), "%02d:%02d", ti->tm_hour, ti->tm_min);
+        int tl = strlen(tb);
+        if (oi + tl < out_len - 1) { memcpy(out + oi, tb, tl); oi += tl; }
+      }
+      p += 6;
+    } else {
+      out[oi++] = *p++;
+    }
+  }
+  out[oi] = '\0';
+}