build(ui): drop sourcemaps from prod bundle

The production UI build shipped `.map` files with full `sourcesContent`, and
the gateway/landing images serve `dist/` verbatim, so anyone could fetch
`/app/assets/main-*.js.map` (same assets under `/vk/`, `/telegram/`) and
reconstruct the entire TypeScript/Svelte source at the edge.

Gate `build.sourcemap` off for `mode === 'production'` (the Docker image build).
Dev and the `mock` e2e build (`vite build --mode mock`) keep maps for debugging.
Document the posture in ARCHITECTURE.md §12.

docs/ARCHITECTURE.md

@@ -1055,6 +1055,13 @@ valid-code population). Brute-forcing a 6-digit friend code within these limits
 accepted MVP risk with low blast radius (an unwanted friendship is removable/blockable);
 a dedicated redeem sub-limit or a longer code is the hardening step if abuse appears.
 
+**Client source exposure.** The production UI build ships **no sourcemaps**
+(`vite.config.ts` gates `build.sourcemap` off when `mode === 'production'`), so the gateway
+and landing images serve only minified JS and the full TypeScript/Svelte source is not
+recoverable from a served `.map` at the edge. Dev and the `mock` e2e build (`vite build
+--mode mock`) keep maps for debugging. This is surface reduction, not secrecy — the single
+minified bundle still carries all platform code paths and is reversible with effort.
+
 ## 13. Deployment (informational)
 
 Single public origin, path-routed. The Vite build has two entries: a lightweight