Calendar view for the queue (#20)

Closes #20 — the signature scheduler UI. A social scheduler without a calendar isn't really one; this adds a month calendar to the queue.

What's added

• List / Calendar toggle in the queue header (choice persisted in localStorage)
• Month calendar placing scheduled (blue dot) and published (green dot) posts in their day cells by local time, with the current day highlighted and prev / today / next navigation
• Click a post chip → edit it (/?edit=<id>)
• Click a day number → compose scheduled for that day: /?schedule=YYYY-MM-DD opens the composer's schedule input pre-set to 09:00 local
• Up to 3 posts per cell with a +N more overflow

No new endpoint — reuses the existing /postline/queue data (scheduledAt, publishedAt, status, title/body).

Verification (browser)

• List is the default; toggling shows the calendar
• Two seeded scheduled posts (14:30 and 09:00 UTC) render on the correct days at correct local times (10:30 PM / 5:00 PM at UTC+8)
• Month nav (next → August, today → July) works; today highlighted
• Day-click → composer with schedule prefilled to …T09:00 and the button reads "Schedule"
• 0 console errors; tsc exit 0; pickier clean

Refs the MVP-readiness set (#17–#20).