Collapse stickies by default — posts and comments

[PortableProgrammer]

Problem

Stickied posts in the feed and stickied comments in threads dominate the view. AutoModerator comments already have a "collapse by default" preference (src/mixins/comment.pug:51) — mirror that for stickies.

Scope

• Sticky posts: in card view, set details[open] to false for p.stickied === true (currently always open for media/link posts in card view per src/mixins/post.pug:57). In compact view, the existing toggle behavior already collapses by default — no change needed.
• Sticky comments: in src/mixins/comment.pug:51, the open= expression currently considers only AutoMod + the collapseAutoMod pref. Extend with && !(data.stickied && prefs.collapseStickies).

Tasks

• Add pref_collapseStickies column to users table (mirror pref_collapseAutoMod)
• DB migration in src/db.js
• Entry in prefToColumn map in src/routes/index.js
• get_user_prefs helper updated
• Dashboard checkbox in src/views/dashboard.pug alongside the existing AutoMod toggle
• Sticky-post collapse logic in src/mixins/post.pug
• Sticky-comment collapse logic in src/mixins/comment.pug