fix: correct manifest splash colors and add required icon sizes

[smartcalsystems-sketch]

Problem

manifest.ts had background_color: "#0a0a0f" (near-black splash) paired with theme_color: "#fbfbfd" (near-white toolbar) — the installed app flashes a dark splash into a white status bar. Icons were only 512x512 and 180x180, with no 192x192 and no purpose: "maskable", so Chrome's install criteria aren't met and Android shows a letterboxed icon.

Fix

• theme_color now matches background_color (both #0a0a0f). A manifest can only hold one static color pair — unlike layout.tsx's viewport.themeColor, which picks light vs. dark per prefers-color-scheme (layout.tsx's viewport metadata has no themeColor, so mobile browser chrome never matches the app's dark/light theme #224) — and #fbfbfd was that light-mode value, not meant for the manifest. Both dark, consistent with the app's dark-mode-first design.
• Added /icon-192.png (plain downscale of the existing icon) and /icon-maskable-512.png for the maskable purpose.

On the maskable icon specifically: I didn't just copy the existing 512px icon and relabel its purpose. Its circle spans roughly 98% of the canvas width, and the W3C maskable spec only guarantees the inner 80% (a centered "safe zone") survives whatever shape an OS crops it into — as-is, the circle's edge would likely get clipped. The new maskable variant scales the existing icon content down to 80% and centers it on a canvas filled with the same background color sampled from the source icon's own corners (#0a0a0f, which already matched background_color exactly), so any OS mask crops only background, never the mark itself. I inspected both generated icons visually before committing them.

Testing

• Updated the existing test that was asserting the old (buggy) theme_color value.
• Added 3 new tests: background/theme color agreement, a maskable 512px icon is present, a 192px icon is present.
• Full manifest.test.ts: 4/4 passing.
• tsc --noEmit and eslint clean on both changed source files.

Closes #695

[vercel]

@smartcalsystems-sketch is attempting to deploy a commit to the chonilius' projects Team on Vercel.

A member of the Team first needs to authorize it.