feat(vscode): VS Code extension — live spend in status bar

[jakduch]

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

A tiny VS Code extension that pins today's Claude Code spend (plus an MTD secondary) to the editor's status bar, sourced from the dashboard's existing /api/data endpoint. Click the indicator to open the dashboard, or run Claude Usage: Refresh Now from the command palette. Polling cadence and dashboard URL are user-settable.

This belongs in the dashboard repo, not a separate one, because it is a zero-context-switch surface for the same data the dashboard already serves. While you are coding — exactly when token spend is being generated — the running cost is one glance away in the status bar; you never alt-tab to a browser tab to remember what today is costing you. The extension is a thin client: no new pricing logic, no new storage, no new API. It mirrors pricing.py in TypeScript only so the status bar can keep rendering during a brief dashboard restart, and a Python structural test guards the layout so dashboard refactors don't silently break it.

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

New files explained: the extension lives entirely under a new vscode-extension/ directory because it is a separately-packaged VS Code artifact (its own package.json, tsconfig.json, and .vscodeignore are required by the VS Code packaging tooling and cannot live at repo root). It adds zero new runtime dependencies to the Python dashboard — devDependencies (typescript, vitest, @types) are TS build/test tooling local to the extension subdir, not deps shipped with the dashboard. A new test file tests/test_vscode_extension.py covers the extension's structural contract from the Python test suite, so a future rename inside vscode-extension/ that breaks the README install instructions will fail python3 -m unittest discover without anyone needing Node locally.