feat: add OpenCode activity and modernize UsageBeam

[oo7kc]

UsageBeam gains optional OpenCode activity, a cleaner native GNOME interface, and a maintainable architecture for the existing provider and collection behavior.

What changed

• Add read-only OpenCode SQLite activity with daily and per-model totals, explicit unsupported states, bounded collection, cancellation, and private derived caching.
• Follow GNOME Shell system surfaces so light/dark themes and configured popup blur style the menu naturally.
• Center the combined UsageBeam/calendar group on its monitor across display sizes, scaling, neighboring center actors, work-area offsets, and RTL allocation.
• Simplify model totals, compact the Activity disclosure, strengthen typography, balance panel padding, and preserve stable panel width across provider/value changes.
• Expand preferences for provider enablement, ordering, defaults, refresh, alerts, retention, and safe derived-data clearing.
• Split provider metadata, usage validation, history cache/reader, snapshots, subprocesses, RPC, refresh policy, and panel presentation into focused modules with enforced dependency boundaries.
• Add development documentation and stricter syntax, named-import, dependency-cycle, schema, layout, and packaging checks.

Validation

• npm run check — 51 Node tests, GJS integration suites, and 9 Python SQLite tests.
• npm run test:stress — 16 synthetic history, cache, notification, subprocess, RPC, bounds, and cancellation cases.
• npm run pack — deterministic allowlisted extension archive.
• Exact-head GNOME smoke passed in native light/dark themes with SF Pro 10 @opsz=17, preferences construction, and disable/re-enable lifecycle.
• Exact-head 1024×600 stress profile passed its UI assertions, 100 refresh/provider/expand cycles, and 10 disable/re-enable cycles.
• Broader display testing across the change covered 1024×600 through 2560×1080, 1×/1.25×/2× scaling, large text, fallback fonts, side docks, RTL allocation, and primary-monitor changes.

No settings keys, usage/cache schema versions, provider token semantics, or extension identity changed.