fix(web): the custom snooze calendar starts the week where the locale does

[Mnigos]

Fixes #12737.

Problem

The custom snooze date picker (Calendar wrapping DayPicker, used only from CustomSnoozeDialog) never set locale or weekStartsOn, so the grid always started on Sunday, DayPicker's en-US default. The trigger label next to it already goes through toLocaleDateString, so the field looked localized while the weekday row did not.

Fix

As suggested in the triage:

• resolveWeekStartsOn(locale) in timestampFormat.ts reads the first weekday from Intl.Locale week info (getWeekInfo() with the older weekInfo getter as fallback) and maps it to a Date#getDay index (week info counts Sunday as 7).
• weekStartsOn is resolved once from the locale timestamps already use: desktopBridge.getSystemLocale() on desktop, since packaged Chromium reports en-US even when the system starts the week on Monday, and the runtime locale in a browser.
• CustomSnoozeDialog passes it to Calendar. When the runtime has no week data the prop is omitted and the current Sunday default stays.

No settings override; the triage says locale is enough for this issue. #9011 (open) makes the "Next week" preset locale-aware with its own helper in client-runtime; this PR does not depend on it, and the two can share one helper once either lands.

Tests

timestampFormat.test.ts: en-US starts on Sunday, en-GB and pl-PL on Monday, ar-EG on Saturday; a malformed locale returns undefined; and with the desktop bridge reporting en-GB the exported weekStartsOn is Monday. 31 tests pass; web typecheck and targeted lint are clean.

Calendar popover in a real browser against the dev server:

Before, en-GB browser	After, en-GB browser	After, en-US browser (unchanged)

Implemented with Claude Code (Claude Fable 5.1).

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All clear

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Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This localized fix changes the custom snooze calendar from its existing Sunday default to a locale-derived week start, with focused tests and limited runtime scope. Because that is a product-default change, it requires human review.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 0ff87f2 and 21e3a7e.

📒 Files selected for processing (3)

• apps/web/src/components/CustomSnoozeDialog.tsx
• apps/web/src/timestampFormat.test.ts
• apps/web/src/timestampFormat.ts

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📝 Walkthrough

Walkthrough

The change resolves the first weekday from locale data and passes it to the custom snooze calendar. Tests cover locale mappings, malformed locales, and desktop locale resolution.

Changes

Locale-aware calendar week start

Layer / File(s)	Summary
Resolve locale week start apps/web/src/timestampFormat.ts, apps/web/src/timestampFormat.test.ts	Adds locale week-data resolution, exports weekStartsOn, converts ISO weekday values to Date#getDay indexes, and tests supported locales and fallback behavior.
Wire week start into the calendar apps/web/src/components/CustomSnoozeDialog.tsx	Passes weekStartsOn to Calendar when the resolved value is available.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: juliusmarminge

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #12737 requires the custom snooze calendar to follow the user locale instead of always starting on Sunday. resolveWeekStartsOn reads locale week data from Intl.Locale, maps ISO weekday value…
Out of Scope Changes check	✅ Passed	The changes are limited to locale week-start resolution, the custom snooze calendar prop, and automated tests for that behavior. These changes directly support issue #12737. No unrelated product behav…
Title check	✅ Passed	The title clearly identifies the main change: the custom snooze calendar now starts the week according to the locale. It is concise and specific despite minor grammatical awkwardness.
Description check	✅ Passed	The description explains the problem, implementation, fallback behavior, tests, and UI results with before-and-after screenshots. It does not use the template headings exactly and omits the checklist,…

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