enhancement: city fields should autocomplete against cities already in the dataset

[shauryagangrade]

Problem

The city field is free text in both places users type one:

• src/components/map/suggest-place-dialog.tsx:140-149
• src/components/map/user-place-dialog.tsx:195-204 (with its own normalizeCity)

Nothing steers contributors toward existing slugs, which is exactly how dataset drift like "new delhi" (space) vs new_delhi happens (see the city-slug hygiene issue). Every variant fragments filtering and city-page generation.

Suggested fix

Autocomplete against getCities(getPlaces()) from src/lib/places.ts — a native <datalist> is enough for v1; a shadcn combobox if we want fuzzy matching. Keep free-text entry allowed for genuinely new cities, but suggest canonical values first.

Acceptance criteria

• Typing a city suggests existing dataset cities
• New cities remain possible without code changes