fix(client): regen all PNG icons as RGBA so macOS Tauri bundler accepts them

Tauri's macOS bundler validates that every PNG referenced from
src-tauri/icons/ is RGBA. Our previous rebuild-icons.py wrote RGB PNGs
because the trim/pad pipeline lives in RGB to keep transparent edges
out of the glassy logo. macOS build failed with:

    error: proc macro panicked
       --> src/lib.rs:1073:16
    message: icon /.../desktop/src-tauri/icons/32x32.png is not RGBA

Fix: scripts/rebuild-icons.py now does `.convert("RGBA")` on every
PNG output before saving. Alpha is fully opaque (255) — the rounded
glass edges retain their look on any background since we paint on
solid white in load_trimmed_square. Same files keep working for
Windows .ico embedding (which accepts RGBA fine).

Regenerate all 17 PNG outputs (32/64/128/128@2x/icon.png + 9 Square*
+ StoreLogo + public/app-icon.png) and the derived .ico/.icns.

Mac side: pull this commit and re-run `bun run build:macos-arm64`.
Windows side: no change in behavior (Tauri's NSIS bundler doesn't
care about PNG color mode).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/rebuild-icons.py

@@ -48,7 +48,13 @@ def main():
     }
     for path, size in sizes.items():
         path.parent.mkdir(parents=True, exist_ok=True)
-        base.resize((size, size), Image.LANCZOS).save(path, "PNG")
+        # Tauri's macOS bundler rejects non-RGBA PNGs ("icon ... is not RGBA").
+        # Force RGBA on every PNG output so the same files work for both
+        # Windows .ico embedding and macOS .icns embedding without per-platform
+        # branches. Alpha channel is fully opaque (255) since we paint on
+        # solid white in load_trimmed_square — the rounded glass edges of the
+        # logo retain their painted look on any background.
+        base.resize((size, size), Image.LANCZOS).convert("RGBA").save(path, "PNG")
         print(f"wrote {path} ({size})")
 
     # Windows Square*Logo / StoreLogo
@@ -66,7 +72,13 @@ def main():
     }
     for name, size in win_sizes.items():
         path = TAURI_ICONS / name
-        base.resize((size, size), Image.LANCZOS).save(path, "PNG")
+        # Tauri's macOS bundler rejects non-RGBA PNGs ("icon ... is not RGBA").
+        # Force RGBA on every PNG output so the same files work for both
+        # Windows .ico embedding and macOS .icns embedding without per-platform
+        # branches. Alpha channel is fully opaque (255) since we paint on
+        # solid white in load_trimmed_square — the rounded glass edges of the
+        # logo retain their painted look on any background.
+        base.resize((size, size), Image.LANCZOS).convert("RGBA").save(path, "PNG")
         print(f"wrote {path} ({size})")
 
     # ICO with multi-resolution.