feat(swa): filter feed to a single thread view

[Ryback2501]

Summary

• Add a thread view by filtering the existing feed to one thread rather than building a separate view. Showing a thread hides every card that doesn't belong to it; going back restores the full feed.

Changes

Feed client (swa/)

• threading.js — add threadRootId (resolves an event's thread root via the root marker, deprecated positional tags, or self) and isInThread membership helper.
• app.js — add activeThreadRoot state; enterThread/exitThread/fetchThread; combine PoW + thread membership in applyFeedFiltersToCard; wire the "Back to feed" banner; reset thread state on tab switch, disconnect, and subscription setup. fetchThread subscribes for the root and its descendants so threads not fully loaded fill in.
• feedView.js — add a "View thread" action button on feed cards.
• index.html — thread banner with a back button.
• style.css — thread banner styling.
• threading.test.js — unit tests for threadRootId and isInThread.

Test plan

• Load the feed, click "View thread" on a reply — only that thread's posts remain visible.
• Click "Back to feed" — the full feed returns.
• Open a thread whose root/replies aren't loaded — they are fetched and appear.
• Switch tabs (Feed/Following/Mentions) while in a thread — thread mode clears.
• Confirm the PoW filter still hides low-difficulty posts inside a thread.