Replace the free-text encounter method input in the route encounter form (RouteEncounterFormModal) with a smarter selector that leverages the known encounter methods already defined in the codebase.
## Current behavior
- The encounter method field in RouteEncounterFormModal is a plain `<input type="text">` with a placeholder "e.g. Walking, Surfing, Fishing"
- Easy to introduce typos or inconsistent naming (e.g. "walking" vs "walk" vs "Grass")
- The app already has a well-defined set of encounter methods in `EncounterMethodBadge.tsx` with METHOD_CONFIG and METHOD_ORDER (starter, gift, fossil, trade, walk, headbutt, surf, rock-smash, old-rod, good-rod, super-rod)
- The backend stores this as a `String(30)` column, so it's not strictly enum-constrained
## Desired behavior
- Replace the free-text input with a dropdown/select that lists the known encounter methods from METHOD_ORDER, using the human-readable labels from getMethodLabel()
- Include an "Other" option that reveals a text input for custom methods not in the predefined list
- When editing an existing encounter, pre-select the correct method
- Consider showing the colored badge preview next to each option for visual consistency with how methods appear elsewhere in the app
## Files
-`frontend/src/components/admin/RouteEncounterFormModal.tsx` — replace the text input with new selector
-`frontend/src/components/EncounterMethodBadge.tsx` — export METHOD_CONFIG or add a helper to get the list of known methods
