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/components/oikos-install-prompt.js

@@ -174,8 +174,8 @@ class OikosInstallPrompt extends HTMLElement {
       .btn-install {
         flex-shrink: 0;
         padding: var(--space-2, 8px) var(--space-4, 16px);
-        background: var(--color-btn-primary, #2554C7);
-        color: #fff;
+        background: var(--color-btn-primary, #4338CA);
+        color: var(--color-text-on-accent, #fff);
         border: none;
         border-radius: var(--radius-sm, 8px);
         font-family: var(--font-sans, system-ui);