feat: draw maidenhead grid on map

[pskillen]

Problem

CodeplugMap plots channel markers and zone hulls but gives no Maidenhead grid context. Operators working with locators (#11 display, #47 standalone tool) cannot visually align map position with grid squares.

Intended outcome

Draw a Maidenhead grid overlay on the existing map component (src/components/CodeplugMap/CodeplugMap.tsx), controlled by a user setting.

Grid rendering

• Overlay field boundaries (4-character / ~2° × 1°) and/or square boundaries (6-character / ~5 km) as map lines.
• Render as a Leaflet layer (e.g. Polyline grid, L.geoJSON, or canvas/grid plugin) that updates on pan/zoom.
• Grid lines should be subtle (low-contrast stroke) so channel markers and zone hulls remain primary.
• Only draw grid lines in the current map viewport (or a sensible buffered bounds) for performance.

Settings

On the Settings page (/settings), add a Maidenhead grid control:

Option	Behaviour
Off	No grid (default)
4-digit	Field grid only
6-digit	Field + square grid

• Persist in browser localStorage (same pattern as tile provider in useMapSettings).
• CodeplugMap reads the setting via useMapSettings (extend hook + Settings.tsx).

Scope of map instances

Apply to all CodeplugMap embeds (channel detail compact map, zone views, any future full-map use) unless a prop override is needed later.

Affected

• src/components/CodeplugMap/CodeplugMap.tsx — grid overlay layer
• New helper, e.g. src/lib/maidenheadGrid.ts — compute line segments for viewport bounds at 4/6-digit precision (reuse rules from src/lib/maidenhead.ts)
• src/hooks/useMapSettings.ts — new setting + localStorage key
• src/routes/Settings.tsx — UI control
• src/components/CodeplugMap/CodeplugMap.md — document overlay behaviour

Notes / dependencies

• Grid math should stay consistent with coordsToLocator / locatorToCoords in src/lib/maidenhead.ts.
• No server calls; purely client-side geometry.
• 8/10-digit subsquare grids are out of scope for this issue.

Out of scope

• Clicking grid cells to set a channel locator (channel edit / feat: tool — maidenhead converter #47 tool territory).
• Grid labels on every cell (optional nice-to-have; not required for v1).

Workflow note

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