feat: "use my location" on maidenhead converter and channel editor

[pskillen]

Problem

Setting a channel or converter location today requires manual coordinate entry, Maidenhead input, map click, or address geocoding. There is no way to seed coordinates from the operator's current device position — useful when programming a hotspot or simplex channel at the current QTH.

The channel edit page (src/routes/channels/edit.tsx) and the Maidenhead converter (src/routes/reference/maidenhead.tsx) both support map pick and (on the converter) address lookup, but neither uses the browser Geolocation API.

Intended outcome

Add a Use my location button that reads the device position and populates the location fields.

Surfaces

Surface	Behaviour
Channel edit (/channels/:id/edit)	Set lat/lon, derive 6-char locator, set useLocation true; move map marker
Maidenhead converter (/reference/maidenhead)	Set lat/lon and locator at current precision; centre MapLocationPicker on result

UX

• Button label: Use my location (or compact variant with crosshairs icon + accessible label).
• Show loading state while navigator.geolocation.getCurrentPosition (or watchPosition with immediate clear) is pending.
• On success: apply coordinates via existing helpers (applyCoords on channel edit; shared coord/locator sync on converter).
• On failure: inline error — permission denied, position unavailable, timeout, insecure context (non-HTTPS). Do not throw; do not block the rest of the form.
• Optional: display reported accuracy (e.g. "±12 m") as dimmed helper text when the browser provides it.

Implementation notes

• Extract a small hook or util, e.g. src/hooks/useGeolocation.ts or src/lib/geolocation.ts, wrapping the Geolocation API with typed errors.
• Reuse coordsToLocator from src/lib/maidenhead.ts — do not duplicate conversion math.
• Channel edit: coordinate with existing location section (applyCoords, map onLocationPick).
• No server round-trip; position never leaves the browser except via existing localStorage persistence when the channel is saved.

Privacy

• Geolocation requires explicit user gesture (button click) and browser permission prompt.
• Document in docs/features/maidenhead.md that device location is used only on demand, not tracked continuously.

Affected

• src/routes/channels/edit.tsx — Location section button
• src/routes/reference/maidenhead.tsx — button near map / coordinates
• New useGeolocation hook or geolocation.ts helper
• docs/features/maidenhead.md — geolocation behaviour
• Optionally src/components/MapLocationPicker/MapLocationPicker.md if button is shared

Notes / dependencies

• Complements map click-to-set (already shipped) and address geocoding on the converter.
• Related: bug: channel edit maidenhead/coords fields drift precision when switched cyclically #49 (maidenhead/coords authoritative field) — if bug: channel edit maidenhead/coords fields drift precision when switched cyclically #49 lands first, "use my location" should set the coords-authoritative path.
• Geolocation may be unavailable in some dev setups (localhost is usually fine; GitHub Pages HTTPS is fine).
• Channel edit does not yet have address lookup (feat: tool — maidenhead converter #47 noted as converter-only); out of scope here.

Out of scope

• Continuous location tracking / watch mode for live updates.
• Setting operator home QTH in Settings as a default for all new channels.
• Mobile-specific UI beyond standard responsive layout.

Manual verify

1. Channel edit → Use my location → lat/lon populate, locator updates, map centres, Use Location checked.
2. Deny permission → clear error message; form still usable.
3. Maidenhead converter → same button → fields and map update.
4. Save channel → coords persist after reload.

Workflow note

Branch from origin/main, atomic conventional commits (hook → channel edit → converter → docs), PR linking Closes #59.