Fix/map share state validation

[techykaif]

What this changes

Fixes #154 by validating shared map viewport parameters before they are passed to Leaflet.

Latitude, longitude, and zoom values are now constrained to valid ranges, with regression tests covering valid boundaries and invalid values.

Type of change

• New public place(s)
• New or updated benefit guide
• New or updated resource link
• Code or fix
• Docs

Proof (required for new public places)

Fill this in for every place added in this PR. It stays in the PR, not in the
dataset.

Field	Value
Source or citation	N/A
Google Maps rating (must be >= 4.0)	N/A
Google Maps review count (must be >= 50)	N/A
Date verified	N/A

• Each place's rating is 4.0 or higher.
• Each place has 50 or more reviews.
• Coordinates point to the correct entrance.
• The JSON record stops at gmaps_link and added_by (no proof fields committed).

Checklist

• The site hosts no copyrighted files; resource and paper changes are links only.
• No em dashes in any copy.

[vercel]

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