Foundation: route, recharts, sprint-dates migration, chart-colors helper (#46)

* feat(ST-1201): add Sprint start_date/end_date + claude_jobs index migration

- Sprint model: optionele start_date en end_date (DATE) voor burndown x-as
- CREATE INDEX claude_jobs(status, finished_at) voor agent-throughput-queries
- Bestaande sprints houden NULL; burndown skipt die

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(ST-1202): add lib/chart-colors.ts + vitest coverage

MD3-token-to-CSS-var mappings for STATUS, PRIORITY, VERIFY, JOB_STATUS
and SERIES_COLORS; all 5 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(ST-1203): add Insights link to NavBar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(ST-1204): move Insights NavBar link between Solo and Todo's

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(ST-1205): add sprint start_date/end_date UI + server actions

- createSprintAction + updateSprintDatesAction: Zod date validation
  with end_date >= start_date cross-check
- start-sprint-button: date inputs in create dialog
- sprint-header: date display button + edit dialog with updateSprintDatesAction
- sprint page: select start_date/end_date for SprintHeader prop
- Demo blokkade via bestaande isDemo checks
- 6 tests groen (validation + demo guard)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -223,6 +223,8 @@ model Sprint {
   product_id   String
   sprint_goal  String
   status       SprintStatus @default(ACTIVE)
+  start_date   DateTime?    @db.Date
+  end_date     DateTime?    @db.Date
   created_at   DateTime     @default(now())
   completed_at DateTime?
   stories      Story[]