fix: refresh relative timestamps after midnight

[agent-p1p]

Fixes #470

Summary

• Refreshes chat and message relative timestamp labels on NSCalendarDayChanged.
• Refreshes after returning to the foreground when a day boundary was crossed while inactive.
• Keeps date formatting out of ordinary chat/message row renders by preserving equatable row boundaries.
• Makes timestamp formatting honor an injected reference date and adds a midnight regression test.

Verification

• git diff --check — passed.
• Conflict-marker sweep — passed.
• Local macOS build/tests unavailable on this Linux worker (xcodebuild and swift are absent).
• GitHub Mac CI will run the Swift format lint, unit suite, release smoke build, and static analysis.

Sensitive paths

None.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6d26e225-7e54-4dea-b6b2-16ff23d644b1

📥 Commits

Reviewing files that changed from the base of the PR and between 0454dbf and 560f641.

📒 Files selected for processing (6)

• whitenoise-mac/ContentView.swift
• whitenoise-mac/Models/MessengerModels.swift
• whitenoise-mac/Views/MessageMediaViews.swift
• whitenoise-mac/Views/MessengerShellView.swift
• whitenoise-mac/Views/SidebarViews.swift
• whitenoise-macTests/whitenoise_macTests.swift

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch pip/whitenoise-mac-470

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[stage-review]

Ready to review this PR? Stage has broken it down into 6 individual chapters for you:

	Title
1	Support reference dates in timestamp formatting
2	Add model methods for relative timestamping
3	Manage reference date in the environment
4	Wire reference date through message views
5	Update sidebar chat row timestamps
6	Verify midnight boundary logic with tests

_{Chapters generated by Stage for commit 560f641 on Jul 12, 2026 3:43am UTC.}

[agent-p1p]

Superseded: this PR accumulated red CI before the shared media-cache test stabilization landed. Reopening the fix as a clean stacked PR on #464; no code from the failing media tests is folded into #470.