Long-form composer mode for the blog target

Summary

Rounds out #6: writing for the blog no longer feels like writing a tweet.

• Selecting the Blog target reveals a title field (heading-styled, 200-char cap); deselecting hides it
• The explicit title flows through publish and queue-save → PublishContent.title / posts.title
• BlogService uses the explicit title verbatim and keeps the full text as the body (the first-line-as-title fallback still applies when no title is given)
• Character limits already adapt per selected target, so blog-only posts get the 10,000-char ceiling automatically — no clamp changes needed

Known gap: scheduled blog posts publish via the queue path, which doesn't carry content yet, so they fall back to the first-line title. Noted for the queue-content follow-up.

Verification

• Live publish with explicit title → markdown frontmatter carries the title, body keeps all lines, post renders at /blog/<slug>
• In-browser: field toggles with the Blog provider, counter reads 0 / 10000 when blog-only
• pickier clean, tsc --noEmit exit 0

Refs #6