Comment sort — per-thread selector and user preference

[PortableProgrammer]

Two coupled additions.

Task 1 — Per-thread sort selector

A sort selector on each comments page allowing the user to re-sort comments (top, best, new, controversial, old, qa) for that thread.

• Add the selector to src/views/comments.pug — model after the existing sort/view dropdowns in src/views/index.pug:45-72
• Plumb a sort param through getSubmissionComments(id, params) in src/geddit.js
• /comments/:id handler at src/routes/index.js:64-97 reads req.query.sort, falls back to user pref, forwards to geddit

Task 2 — User preference for default comment sort

Separate from the feed sort preference, applied automatically when entering any comments page. The per-thread selector overrides for that page only.

• New DB column pref_commentSort (default confidence — Reddit's "best")
• Migration in src/db.js
• Entry in prefToColumn map in src/routes/index.js
• get_user_prefs helper updated
• Dashboard selector in src/views/dashboard.pug

Acceptance

Changing comment sort on one thread persists for that thread only via URL param; the user pref controls the default on all threads.