feat: config-driven region (studymap.config.ts)

[AnayDhawan]

Headline v2.0 work (self-host / fork-loop). Region settings are baked into code, so a fork can't
retarget a new city without editing components. This is what lets a student in any city run StudyMap
for their own region.

Task

• Create studymap.config.ts at the repo root holding center, defaultZoom, bounds, and the
  cities registry, sourced from src/lib/constants.ts.
• Make the map, filters, and data validation read from this config.

Acceptance: changing only studymap.config.ts retargets the map to a different city/region.
Depends on: the constants-extraction keystone (v1.1.0).