feat: keep the map viewport in the URL so a place can be shared by link

[aryansk]

Problem

Closes #118. Sharing a specific place or view wasn't possible: opening /map always landed on the default view, and reloading lost the selected place and viewport.

What already existed (kept intact)

• ?place=<id> selects the place, opens its card, and flies to it (via FlyToPlace); ?place=nonsense falls back to the default view.
• city / types filter params round-trip through the URL.

What this PR adds

• Viewport params: lat, lng, zoom. On load they restore that exact center/zoom instead of fitting all places.
• URL stays in sync: a ViewportReporter inside the map mirrors pan/zoom end into the URL with history.replaceState — the acceptance criteria's "panning updates the URL but adds no browser history entries", so the back button still exits in one press.
• Copy link reproduces the view: the map's share button and the pin popup's "Copy link" now include the viewport (pin links center on the place at zoom 15).
• Robust parsing: lat/lng only apply when both are present and finite; a missing or bogus pair (?lat=abc) degrades to the default view without crashing.

Verification

• 42 unit tests pass (11 in share.test.ts, including viewport round-trips, zoom-omission, and malformed-param handling).
• tsc --noEmit, eslint, and next build clean.
• next start smoke test: /map, /map?lat=19.076&lng=72.8777&zoom=12, /map?place=mum-library-01, /map?place=nonsense, and /map?lat=abc&lng=72.8777 all return 200 with no server errors.

[vercel]

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