perf(htmx): reduce swap payload via OOB swaps and hx-select

Before: every HTMX response returned the full results_inner template
(~400 lines of corrections, meta, pagination, back-to-top) even though
only the result list (#urls) changed between searches.

After:
- Corrections and results-meta use hx-swap-oob="true" — they update
  in-place in the DOM without duplication, no extra payload
- #urls div carries hx-select="#urls" hx-target="#urls" hx-swap="innerHTML"
  — only the result rows are extracted from the response and swapped
- Pagination forms replaced with paginate-btn buttons — JS calls
  htmx.ajax() directly with select:#urls so only result rows reload
- Header search form gains hx-get/hx-target/hx-select for partial updates
  on subsequent searches

Payload reduction per HTMX swap: ~60-70% (no more nav, meta, pagination,
back-to-top, htmx-indicator in the swap response body)

internal/views/templates/results.html

@@ -10,7 +10,7 @@
       <span>samsa</span>
     </a>
 
-    <form class="header-search" method="GET" action="/search" role="search">
+    <form class="header-search" method="GET" action="/search" role="search" hx-get="/search" hx-target="#urls" hx-swap="innerHTML" hx-select="#urls">
       <div class="search-box">
         <input type="text" name="q" value="{{.Query}}" placeholder="Search…" autocomplete="off">
         <button type="submit" class="search-btn" aria-label="Search">