Usage: pick This month or an earlier calendar month

[Ramanan-Bala]

Problem

Usage only offers rolling windows: Past 24h, 7 days, 30 days and 90 days. There is no way to see the current calendar month ("how much have I spent this month?") or a past month ("what did October cost?"). "30 days" is close but never lines up with a month.

Proposal

Add a month option to the Usage period control:

Past 24h · 7 days · 30 days · 90 days · [ Select month ▾ ]

• The last segment reads Select month while a rolling period is active. It opens a short menu: This month (month to date, with its range such as "Nov 1 – 8"), then each earlier month that has complete data, such as October 2026 and September 2026.
• After a past month is picked, the segment shows its name ("October ▾") and the line under the total shows the range ("Oct 1 – 31 · 43 sessions · API estimate").
• Mod+Shift+5 selects This month, next to the existing Mod+Shift+1–4. The existing usage.period.month command stays bound to 30 days.
• Mobile gets a "Month" segment plus a native month menu.

Scope and constraints

• No contract or server logic change. UsageSummaryInput already carries sinceDay/untilDay and the viewer's timeZone, and the server already groups by calendar day in that zone, so a month is just a different range computed on the client.
• Only months with complete data are listed. The server keeps saved per-file usage for 92 days (CACHE_RETENTION_DAYS), and providers delete old transcripts (Claude Code after 30 days by default). The menu lists only months whose first day falls inside that window, so a listed month is never silently partial. That 92-day value becomes one shared constant so the server and clients can't disagree. Going back further, by keeping data longer, is deliberately out of scope.
• Persistence: This month is remembered like the other periods. A specific past month is not; reopening Usage shows This month.
• No custom date-range picker.

Implementation

A working implementation is on a branch: https://github.com/Ramanan-Bala/t3code/tree/feat/usage-month-picker. The matching PR is #32824. I'm happy to change the scope or UI based on feedback, or to close the PR if this isn't a direction you want.

Screenshot

This month selected with the month menu open: