PWA: generate manifest icon assets (192/512/maskable)

[deanjstone]

Extend scripts/export-brand-icons.ts to also emit 192×192 and 512×512 PWA icons plus a maskable variant, output into apps/web/public. Reuse the existing brand icon source/pipeline rather than a one-off script.

Part of map #1.

[deanjstone]

Extended the existing brand-icon pipeline (scripts/export-brand-icons.ts + scripts/lib/brand-assets.ts) rather than a one-off script:

• New VariantOutputs fields pwaIcon192, pwaIcon512, maskableIcon512, rendered per brand (dev/nightly/prod) via the same Icon Composer iOS platform export already used for the other web icon sizes.
• Maskable variant reuses the 512×512 render (same pattern already used for ios/universal sharing one buffer) — no separate safe-zone composition step exists in this pipeline yet.
• resolveWebIconOverrides now also copies pwa-192x192.png, pwa-512x512.png, and maskable-icon-512x512.png into apps/web/public (dev) and dist/client (publish), same as the existing favicon/apple-touch-icon overrides.
• Added/updated tests in brand-assets.test.ts; vp test run (147 passed) and tsgo --noEmit both clean on @t3tools/scripts.

Note: actually regenerating the PNGs requires Icon Composer, which is macOS-only — same constraint the existing icons already have. This machine (WSL2) can't produce the real bytes, so assets/{dev,nightly,prod}/*-pwa-*.png / *-maskable-icon-*.png don't exist on disk yet; npm run icons:export needs to run on a Mac (or via the existing Codex/Icon Composer prompt path) before #3's manifest.json can reference real files.

Commit: d9d87d2 on feature/pwa-web-app.