ci(deploy): switch from Cloudflare Pages to GitHub Pages

Migrate build and deployment pipeline from Cloudflare Pages to GitHub Pages.
Adds production-ready deploy workflow in deploy-github-pages.yml with proper
artifact handling. Removes Cloudflare-specific tooling: wrangler config, _headers,
_redirects, and CSP hash injection scripts (no longer needed with GitHub Pages
static hosting). Updates package.json build scripts and all project documentation
to reflect new deployment target and simplified architecture.

web/static/_headers

@@ -1,14 +0,0 @@
-/*
-  Content-Security-Policy: default-src 'self'; img-src 'self' data:; media-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self' 'unsafe-inline'; connect-src 'self'; font-src 'self' data:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'; manifest-src 'self'; worker-src 'self'
-  X-Content-Type-Options: nosniff
-  Referrer-Policy: strict-origin-when-cross-origin
-  Permissions-Policy: accelerometer=(), camera=(), geolocation=(), gyroscope=(), microphone=(), payment=(), usb=()
-  X-Frame-Options: DENY
-
-# Service worker must always re-validate so fresh deploys propagate.
-/sw.js
-  Cache-Control: no-cache
-
-/manifest.webmanifest
-  Cache-Control: no-cache
-  Content-Type: application/manifest+json

web/static/_redirects

@@ -1,10 +0,0 @@
-# Cloudflare Pages routing rules.
-#
-# 1. /master* → 301 redirect to / (handles old bookmarks from when the
-#    host had its own page; the URL bar updates to /).
-# 2. /* → 200 rewrite to /index.html (SPA fallback for any other unknown
-#    path: keeps the URL but serves the homepage content). Static assets
-#    take precedence over rules, so /, /_app/*, /favicon.ico, etc. are
-#    served directly without rewrite — no redirect loop on /.
-/master*  /             301
-/*        /index.html   200