feat(analytics): make period selector functional with api-level filtering

[Mohammad-Hassan027]

Description

The Analytics page had a period selector (This Week / This Month / All Time)
that was purely decorative — the fetch always called /api/user/stats
with no query parameter, so the stats cards and activity chart never
changed regardless of selection.

This PR wires up the selector end-to-end:

API changes (app/api/user/stats/route.ts):

• Added a parsePeriod() helper that validates the ?period= query
  param (week | month | all, defaults to week).
• GET() now accepts NextRequest and reads the period from the URL.
• Computes a periodStart cutoff date and filters all data into
  period-scoped subsets.
• Returns period-scoped values for learningHours, quizzesTaken,
  aiChats, flashcardSessions, and weeklyActivity.
• Study streak intentionally stays all-time — it is a cumulative metric
  and filtering it by period would produce misleading results.

Page changes (app/(dashboard)/analytics/page.tsx):

• Stats useEffect now depends on [period] and fetches
  /api/user/stats?period=${period}.
• setIsLoading(true) is called before each refetch so skeletons
  appear while new data loads.
• Fixed a pre-existing dataKey="day" → dataKey="label" bug on the
  Weekly Activity chart XAxis (the API always returned label).

Related Issue: Closes #91

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (non-breaking change to add/edit documentation)

How Has This Been Tested?

• Period selector — Week: Selected "This Week", confirmed stats
  cards and activity chart reflect the last 7 days of data only.
• Period selector — Month: Selected "This Month", confirmed
  stats reflect current calendar month and the chart switches to a
  monthly bar view.
• Period selector — All Time: Confirmed all-time aggregates are
  returned and the monthly chart is shown.
• Loading state: Confirmed skeleton placeholders appear between
  period switches before data resolves.
• TypeScript: Ran npx tsc --noEmit — no new errors introduced
  in analytics/page.tsx or api/user/stats/route.ts.

Visual Proof (Screenshots/GIFs/Videos):
N/A — UI layout is unchanged; only the data displayed updates on
period selection. Manual verification against a seeded account
is recommended.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings/lint errors
• I have checked that my branch is up-to-date with main
• All tests pass locally with my changes

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