fix(submit): preserve per-device cross-machine breakdowns

[IvGolovach]

Preserve per-device source breakdowns when the same user submits usage from multiple machines.

The existing merge path can overwrite a client's daily source breakdown when a different device re-submits the same day. This change stores per-device contributions inside each client bucket, replaces only the current device's contribution on re-submit, and preserves other devices.

It also migrates legacy client-only entries into a reserved __legacy__ device bucket, and reconciles days that disappear from the current device's submitted date range without touching days outside that range.

No database migration is required.

Related to #248.

Checks:

• cd packages/frontend && ../../.bun/bin/bun x vitest run __tests__/api/submit.test.ts
• cd packages/frontend && ../../.bun/bin/bun x vitest run
• cd packages/frontend && ../../.bun/bin/bun x tsc --noEmit

[vercel]

@IvGolovach is attempting to deploy a commit to the Inevitable Team on Vercel.

A member of the Team first needs to authorize it.

[cubic-dev-ai]

1 issue found across 4 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="packages/frontend/src/app/api/submit/route.ts">

<violation number="1" location="packages/frontend/src/app/api/submit/route.ts:274">
P1: Disappeared-day reconciliation prunes only `submittedClients`, allowing stale current-device data to persist for omitted clients.</violation>
</file>

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