aw-notify: achievements / streaks / trophies (split from #200)

Splitting this out from ActivityWatch/aw-android#200, per @ErikBjare's suggestion — the achievements/streaks/trophies direction is a distinct enough feature to track on its own rather than living inside the broader "smarter-than-thresholds" notifications thread.

## Background

In #200 we compared anomaly-detection notifications (percentile/sigma above baseline) against a "coaching"/reward angle, and landed on achievements as a promising first step:

- Works with minimal history (a first occurrence is still a valid achievement) — unlike anomaly detection, which needs ~30 days of baseline before it's useful.
- Framing is additive/positive rather than corrective.
- "Trophy for best focus day this month" is more shareable/engaging than "you're 2σ above baseline."
- Fits the existing notification infrastructure (per-category historical time from aw-server) either way.

References:
- https://dl.acm.org/doi/10.1145/2858036.2858428 (ACM CHI — self-quantification + gamification, solid UX evidence)
- https://forum.activitywatch.net/t/achievements-progress-bars-streaks-and/1630 (existing community appetite)
- https://activitywatch.net/blog/future-of-time-tracking/ ("meaningful milestones" framing)

## Open design questions (from @ErikBjare, carried over from #200)

- **Storage**: how do achievements get stored once conditions are met? Need references to which data sources were used, the time-range covered, and the source devices/buckets involved — not just a flat "achievement unlocked" flag.
- **Multi-device**: should each device have its own achievement store that overlays locally, or should achievements sync across devices?
- Sync as part of "settings"? Is settings-sync even supported yet?
- Or should achievements live in their own dedicated bucket (e.g. an `achievements` bucket)? Is that a good fit for the existing bucket/event model, or a bad one?
- Trophies should be collectable multiple times (e.g. per time-period — "best focus day this month" can recur), not one-shot unlocks.

## Scope (not yet decided)

This issue is for continuing the design discussion — no implementation approach is locked in yet. Candidate shape: define achievement conditions declaratively, evaluate them against aw-server query results, surface via aw-notify, with the storage/sync model above still open.

cc @ErikBjare