feat: cross-platform tray / menu-bar app showing today's spend

[jakduch]

What does this add and why do you believe it belongs in this dashboard?

This adds a cross-platform tray / menu-bar companion that surfaces today's Claude Code spend right in the system tray (Linux/Windows) or macOS menu bar, with a single click to open the full dashboard in the browser.

The case for it fitting a personal Claude Code usage dashboard: cost awareness only changes behaviour when it is glanceable. Opening the dashboard is a deliberate act — by the time a heavy day is noticed, the spend has already happened. A persistent indicator turns that into ambient feedback: the running dollar figure is always in the corner of the screen, colour-coded green / amber / red against the existing $1 and $10 thresholds, and refreshed every 60 seconds from the dashboard’s own /api/data endpoint. No new server, no new data path, no duplicate pricing logic — it reuses pricing.calc_cost and the running dashboard as the single source of truth. Clicking the indicator opens the dashboard for the full drill-down. Both backends (rumps on macOS, pystray elsewhere) are optional and lazy-imported, so nothing changes for users who don’t opt in.

Checklist

Code correctness

• All calcCost() calls pass 6 arguments: (model, inp, out, cache_read, cache_creation, cache_1h)
• JavaScript template literals use bare backticks (`), not escaped ones (\`)
• No JS variables referenced before they are defined
• No new third-party dependencies introduced

Tests

• python3 -m unittest discover -s tests -v — all passing
• python3 -m unittest tests.test_browser -v — all passing
• New behaviour is covered by at least one test

Scope

• This is a single concern — one feature or fix per PR
• Only touches existing files (dashboard.py, scanner.py, cli.py, pricing.py, cowork.py, tests/) — or I’ve explained below why a new file is needed

tray.py is a new top-level module. It is isolated by design: the tray UI is an optional, lazily-loaded companion process with its own platform backends (rumps / pystray), and putting it next to cli.py keeps dashboard.py and scanner.py free of GUI concerns. tests/test_tray.py is added alongside it. No third-party imports are added at module top level — both GUI libraries are imported inside run() so import tray always succeeds (CI, headless dev, users who never enable it).