bug: channel edit maidenhead/coords fields drift precision when switched cyclically

[pskillen]

Problem

On the channel edit page (#11), Maidenhead locator and lat/lon coordinate fields are both active at once. Editing one continuously updates the other:

• Changing lat/lon calls coordsToLocator(lat, lon, 6) on each keystroke (edit.tsx NumberInput onChange handlers).
• Blurring the locator field calls locatorToCoords via applyLocator, snapping to the centre of the 6-character square.

Cycling locator → coords → locator → coords incrementally reduces coordinate precision because each round-trip quantises to the Maidenhead cell centre at 6-char resolution (~5 km). A user entering precise GPS coordinates will see them drift if they touch both fields.

Root cause

Two authoritative inputs kept in sync bidirectionally. Maidenhead is a lossy representation at fixed precision; treating both as live sources of truth causes drift.

Intended fix

UI behaviour

• One active input mode at a time: either coordinates (lat/lon) or Maidenhead locator — not both editable simultaneously.
• Switch control: button or segmented control to change mode, with options:
  • Switch without converting (discard/sync the inactive field)
  • Switch with converting (populate the target from the current authoritative value)
• Convert button: explicit one-shot "locator → coords" or "coords → locator" while staying in the current mode (optional; may be folded into switch-with-convert).
• Detail page: display both read-only values derived from the stored authoritative source (no drift on view).

Data model

Add explicit storage for the operator's chosen representation:

• New field on Channel, e.g. maidenheadLocator: string | null (precision as entered, normalised uppercase).
• Mutual exclusivity: when maidenheadLocator is set, location is null (or vice versa). Only one is authoritative in the model.
• useLocation still gates whether location data is active at all.
• Update docs/features/data-model/ and schema version / migration for existing projects (default: coords-only from current location field).

Import / export

• OpenGD77 import: unchanged — CPS provides Latitude / Longitude only; populate location, leave maidenheadLocator null.
• OpenGD77 export: CPS expects coordinates — if maidenheadLocator is authoritative, convert via locatorToCoords at export time and emit lat/lon; if location is authoritative, emit as today.
• No Maidenhead column in OpenGD77 CSV today.

Repro

1. Edit a channel; enter precise coords (e.g. 55.953252, -3.188267).
2. Tab to locator (auto-filled 6-char, e.g. IO85 + subsquare).
3. Blur locator (applies locatorToCoords → centre of cell).
4. Edit lat/lon slightly → locator updates → blur locator again.
5. Observe coords have drifted from original precision.

Affected

• src/models/codeplug.ts — maidenheadLocator (or equivalent) + migration
• src/routes/channels/edit.tsx — mutually exclusive input UX
• src/routes/channels/detail.tsx — read-only display from authoritative field
• src/lib/import/ / export layer — coords conversion at boundary
• docs/features/maidenhead.md, docs/features/data-model/

Notes / dependencies

• Introduced in feat: CRUD channels #11 channel edit form.
• Conversion math stays in src/lib/maidenhead.ts — do not fork.
• Standalone maidenhead tool (feat: tool — maidenhead converter #47) is separate; this fixes channel persistence semantics.

Out of scope

• Storing both coords and locator as independently authoritative (that recreates the bug).
• Sub-6-char locator precision in the model unless explicitly entered by the user.

Workflow note

Branch from origin/main, atomic conventional commits, PR linking Closes #.