Add TinySearch-powered site search

[mre]

Summary

• add a shared search and theme control to homepage and post headers
• build live full-text results with TinySearch 0.11.0 and same-origin navigation
• keep the search form in normal document flow and replace page content while searching
• include the generated WASM for plain Zola local previews
• update the production build and CI to generate the latest search index

Validation

• zola build
• TinySearch 0.11.0 optimized WASM build
• desktop and mobile headless Chrome interaction checks
• local-origin result navigation check
• JavaScript syntax and project diagnostics