Fix silently missing titles for pages with a late <title>

Fix silently missing titles for pages with a late <title>

fetch_page() only ever read the first 8 KB of a response, so any page
whose <title> sits beyond that (github: byte 24571) produced no title at
all -- the log showed "fetching page ..." and then nothing, and the
plugin published an empty action. Read in chunks until </title> shows
up instead, capped at 512 KB, and skip the body entirely for non-text
responses.

Also along that path:
- decode with errors='replace'; the read boundary can land mid-character
- extract_title() passed user_agent=None, and requests treats a None
  header value as "drop this header", so every non-YouTube fetch went
  out with no User-Agent at all
- the non-text branch returned the tuple (1, content-type), which is
  truthy, so URLResolver called .strip() on it and raised AttributeError
- log a warning on the remaining no-title paths, which were silent
- return None instead of Action(msg="") when nothing resolved

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>