fix: smaller audit items — strnlen, narrow-display guard, SNR precision

M2: CMD_SET_DEFAULT_FLOOD_SCOPE used strlen() on the frame's 31-byte name
slot, which is not required to be NUL-terminated. Switched to strnlen()
so the search can't run past the field into the 16-byte key.

M4: NearbyScreen::renderDiscoverDetail computed strncpy(b64, ..., max_chars - 3)
where max_chars came from display width. On very narrow displays (width < ~28
at 6 px font) this became negative. Skip the pub-key line entirely when
max_chars < 4 so we don't risk a negative count and a bogus terminator.

L1: SNR was shown as truncated integer dB. Switched the detail view and
the 2-line discover cards to %.1f so they keep the 0.25 dB resolution
(consistent with the ping popup, which already used %.1f).

L4: Two fallback "?" sender placeholders used strncpy(buf, "?", sizeof(buf))
— functional but it memsets 21 unused bytes for a one-character string.
Replaced with strcpy.

M1 marked as not-a-bug after re-check: default_scope_name is char[31],
so the n < 31 guard correctly admits the max 30-char string + NUL.

FEATURES.md audit section updated with current status.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/companion_radio/ui-new/QuickMsgScreen.h

@@ -784,7 +784,7 @@ public:
             strncpy(fsender, ftext, nl); fsender[nl] = '\0';
             strncpy(fmsg, fsep + 2, sizeof(fmsg) - 1); fmsg[sizeof(fmsg)-1] = '\0';
           } else {
-            strncpy(fsender, "?", sizeof(fsender));
+            strcpy(fsender, "?");
             strncpy(fmsg, ftext, sizeof(fmsg) - 1); fmsg[sizeof(fmsg)-1] = '\0';
           }
           int ret = _fs.render(display, fsender, fmsg,
@@ -857,7 +857,7 @@ public:
           strncpy(msg_part, sep + 2, sizeof(msg_part) - 1);
           msg_part[sizeof(msg_part) - 1] = '\0';
         } else {
-          strncpy(sender, "?", sizeof(sender));
+          strcpy(sender, "?");
           strncpy(msg_part, text, sizeof(msg_part) - 1);
           msg_part[sizeof(msg_part) - 1] = '\0';
         }