Fix timeframe sorting dropdown not reloading page

[ahmedsicp]

What

When visiting a subreddit and sorting by Top or Controversial (such as /r/reddit/top), picking an option from the timeframe dropdown (Hour, Day, Week, Month, Year, All) does not reload the page or update the URL to ?t=<timeframe>. Users currently have to manually click the tiny arrow submit button next to the dropdown for the timeframe filter to apply.

Root cause

1. The <select id="timeframe"> dropdown had no change event listener or onchange handler, so choosing an option only mutated the <select> value in the DOM without submitting the parent form.
2. The <form id="sort"> did not specify an explicit method="get" and action attribute pointing to the active sort listing (/r/{sub}/top or /top).
3. Redlib's Content Security Policy (script-src 'self' blob:;) disallows inline script execution, so inline onchange="..." handlers alone are blocked by the browser unless accompanied by a script served from 'self'.

Changes

• CSP-compliant client script (static/sort.js): Added a small script registered in src/main.rs as /sort.js and loaded with defer in templates/base.html. It listens for change events on #timeframe, #sort_select, #commentSortSelect, and #search_sort select[name="sort"] and submits the form upon selection.
• Form actions & methods: Added explicit method="get" and action to sort forms in templates/subreddit.html, templates/user.html, templates/search.html, and templates/post.html, along with an onchange="this.form.submit()" inline fallback.
• Non-JS fallback preserved: The <button id="sort_submit"> button is kept in place so users browsing with JavaScript disabled can still submit the form manually.
• Unit test: Added test_subreddit_sort_form_render in src/subreddit.rs to verify that SubredditTemplate renders the form with the expected action, onchange handler, and script tag.

Verification

• Built and tested with cargo check and cargo test.
• Ran a local Redlib instance and verified:
  • Visiting /r/reddit/top and selecting a timeframe submits the form and reloads at /r/reddit/top?t=<timeframe>.
  • Same behavior verified for /top, multireddits (/r/rust+golang/top), /r/reddit/controversial, /user/:name, and /search.
  • Verified /sort.js serves HTTP 200 with content-type: text/javascript.