fix: icon centering — use Center gravity with signed offsets

Previous script used NorthWest gravity with fixed pixel offsets, placing
the BR pair in the upper-left quadrant. When Android clips to a circle
the letters appeared off-center.

New approach: -gravity Center with ±HALF offsets so each letter is
symmetrically placed around the canvas centre mathematically. No
subshells, no pixel guesswork. Regenerated all four icon assets.

scripts/generate_icons.sh

@@ -1,7 +1,11 @@
 #!/usr/bin/env bash
 # Generates all icon assets for bincio-rec.
-# Requires ImageMagick (magick / convert).
-# Style mirrors bincio_autarchive: dark background, white B, accent-coloured R.
+# Requires ImageMagick 7 (magick).
+#
+# Centering strategy: both letters are rendered with -gravity Center.
+# Each letter's centre is offset ±HALF pixels from the canvas centre,
+# where HALF = (letter_width + gap) / 2.  This gives mathematically
+# correct horizontal & vertical centering regardless of font metrics.
 
 set -e
 cd "$(dirname "$0")/.."
@@ -11,33 +15,37 @@ BG="#09090b"
 WHITE="#ffffff"
 RED="#ef4444"
 
-# ── Main icon (1024×1024) ─────────────────────────────────────────────────────
-magick -size 1024x1024 xc:"$BG" \
-  -font "$FONT" \
-  -pointsize 480 -fill "$WHITE" -gravity NorthWest -annotate +110+295 'B' \
-  -pointsize 480 -fill "$RED"   -gravity NorthWest -annotate +560+295 'R' \
-  assets/icon.png
+# render_br CANVAS BG SIZE HALF OUT
+# HALF = how many pixels each letter centre is offset from canvas centre.
+render_br() {
+  local CANVAS=$1 BG_COL=$2 SIZE=$3 HALF=$4 OUT=$5
+  magick -size "${CANVAS}x${CANVAS}" xc:"$BG_COL" \
+    -font "$FONT" -pointsize "$SIZE" -gravity Center \
+    -fill "$WHITE" -annotate "-${HALF}+0" 'B' \
+    -fill "$RED"   -annotate "+${HALF}+0" 'R' \
+    "$OUT"
+}
 
-# ── Android adaptive icon — foreground (transparent bg, design fits safe zone)
-# Safe zone = inner 66% of 1024 = 676 px.
-magick -size 1024x1024 xc:none \
-  -font "$FONT" \
-  -pointsize 340 -fill "$WHITE" -gravity NorthWest -annotate +175+325 'B' \
-  -pointsize 340 -fill "$RED"   -gravity NorthWest -annotate +535+325 'R' \
-  assets/android-icon-foreground.png
+# render_br_transparent CANVAS SIZE HALF B_FILL R_FILL OUT
+render_br_transparent() {
+  local CANVAS=$1 SIZE=$2 HALF=$3 B_FILL=$4 R_FILL=$5 OUT=$6
+  magick -size "${CANVAS}x${CANVAS}" xc:none \
+    -font "$FONT" -pointsize "$SIZE" -gravity Center \
+    -fill "$B_FILL" -annotate "-${HALF}+0" 'B' \
+    -fill "$R_FILL" -annotate "+${HALF}+0" 'R' \
+    "$OUT"
+}
 
-# ── Android adaptive icon — monochrome (white on transparent, for themed icons)
-magick -size 1024x1024 xc:none \
-  -font "$FONT" \
-  -pointsize 340 -fill "$WHITE" -gravity NorthWest -annotate +175+325 'B' \
-  -pointsize 340 -fill "$WHITE" -gravity NorthWest -annotate +535+325 'R' \
-  assets/android-icon-monochrome.png
+# At 480 pt NotoSans-Bold, each capital is ~320 px wide.
+# Gap between inner edges: 60 px.  Inter-centre: 320 + 60 = 380 → HALF = 190.
+render_br            1024 "$BG"  480 190 assets/icon.png
 
-# ── Splash screen icon (512×512) ─────────────────────────────────────────────
-magick -size 512x512 xc:"$BG" \
-  -font "$FONT" \
-  -pointsize 240 -fill "$WHITE" -gravity NorthWest -annotate +55+147 'B' \
-  -pointsize 240 -fill "$RED"   -gravity NorthWest -annotate +280+147 'R' \
-  assets/splash-icon.png
+# Adaptive foreground (transparent, fits the 66% safe zone).
+# 320 pt → letter ~215 px wide.  Gap 50 px → HALF = 132.
+render_br_transparent 1024 320 132 "$WHITE" "$RED"   assets/android-icon-foreground.png
+render_br_transparent 1024 320 132 "$WHITE" "$WHITE" assets/android-icon-monochrome.png
+
+# Splash 512×512 — 240 pt → letter ~160 px wide.  Gap 36 px → HALF = 98.
+render_br            512  "$BG"  240 98  assets/splash-icon.png
 
 echo "✓ Icon assets generated in assets/"