chore: complete colour-redesign follow-up — PWA theme sync, glass tokens, print normalisation (v0.20.16)

- index.html: theme-color #2563EB → #4F46E5 (Indigo-600)
- oikos-install-prompt.js: CSS fallback #2554C7 → #4338CA; #fff → var(--color-text-on-accent)
- tokens.css: add --glass-inset-{soft,base,medium,elevated,strong} tokens
- glass.css: replace 9 inset rgba() literals with --glass-inset-* token refs
- tasks.css: replace 1 inset rgba() literal with --glass-inset-base
- layout.css: normalise @media print shorthand hex to six-digit notation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/tasks.css

@@ -133,7 +133,7 @@
     backdrop-filter: var(--blur-xs);
     -webkit-backdrop-filter: var(--blur-xs);
     border-color: color-mix(in srgb, var(--color-accent) 50%, transparent);
-    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.20);
+    box-shadow: var(--glass-inset-base);
   }
 }