Add "this week" date filter option to insights

Feature request

Is your feature request related to a problem?

PostHog currently offers "today", "this month", and "last week" as date filter options, but there's no "this week" option available. This creates an inconsistency in the date picker and makes weekly reporting more difficult than it needs to be.

For example:

• To see this month's data, I can select "this month" ✅
• To see today's data, I can select "today" ✅
• To see last week's data, I can select "last week" ✅
• To see this week's data, I have to use "last 7 days" (which is a rolling window, not the current calendar week) ❌

This makes it difficult to:

• Review weekly performance in team meetings (we want "this week", not "last 7 days")
• Compare "this week" vs "last week" consistently
• Have symmetry between current and past period reporting

Describe the solution you'd like

Add a "this week" option to the date picker that shows data from the start of the current calendar week through now (respecting the "week starts on" setting in project preferences).

This would create a consistent pattern:

• Today (exists) / Yesterday
• This week (missing) / Last week (exists)
• This month (exists) / Last month

Example use cases:

• Sales teams reviewing this week's pipeline activity
• Support teams tracking this week's ticket resolution
• Product teams monitoring this week's feature engagement

Describe alternatives you've considered

Current workarounds:

• Using "last 7 days" - but this is a rolling 7-day window, not the current calendar week
• Manually selecting the start of the week to today - time-consuming and needs to be updated daily
• Creating SQL insights with custom date logic - not accessible to non-technical users

Why "last 7 days" isn't the same:

• On a Wednesday, "last 7 days" includes Thursday-Wednesday, not Monday-Wednesday
• It doesn't align with how teams naturally think about weeks
• It can't be directly compared to "last week" for week-over-week analysis

Additional context

Why this is needed:

• Symmetry: We have "last week", so we should have "this week"
• Consistency: We have "today" and "this month" for current periods
• User expectation: When users see "this month", they expect "this week" to exist

Technical note:

PostHog already has the infrastructure for this:

• Calendar week logic exists (used in "last week")
• "Week starts on" setting exists in project preferences
• The date picker already supports calendar-based periods

This is a small but high-impact addition that would complete the calendar-based date filter options.

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]