feat: CRUD channels

[pskillen]

Problem

Channels can only be viewed, not created, edited, or deleted.

Intended outcome

Full create / read / update / delete for channels against the internal data models.

• Add, edit, and delete channels.
• Validate inputs (frequencies, location/coordinates, channel type, DMR contact/TG references).
• Location tools - pick on map, address/postcode lookup, maidenhead (2 way - input, and display)

Other nice features to add to details/input page:

• Bands - consistent colour mapping for band, pill on channel details page, pill in list table (e.g. 2m, 70cm, 40m)
• Show rx/tx frequency and offset (e.g. +7.6MHz). Allow input in either format.
• Hide inputs not relevant to mode (e.g. digital - hide ctcss/dcs tone)
• on the details page, hide the "draw zones" and "label..." checkboxes because they don't make sense in a single station context.

Nice features for data table:

• Highlight digital/fm/am/ssb channels (colour-consistent pill?)
• Replace location "yes/no" with Maidenhead locator or [blank]
• Show/hide cols settings - persist in localstorage. Name/mode/tx/rx probably should always be visible.

Filters to add to data table

• name (free text)
• band
• simplex/duplex (i.e. tx=rx freq, or split)
• Digital / FM / AM / SSB (USB or LSB)

Data model

• We probably need a callsign and a display name field - these already exist but the latter is just name
• Persistent option, per channel, to prepend callsign on export (e.g. yes/no/last 2 letters)
• add a field called "hide from map" - this field will be internal to this tool (i.e. no export), but I have a very specific use case: most of my zones have my hotspot as a member. When I visit Aberdeen the hotspot comes with me, so it's no longer at its GPS location, but since it's in the Aberdeen zone it makes that zone's convex hull stretch from Aberdeen to Glasgow on the map.

[pskillen]

Implementation started on branch 11/paddy/channels-zones-crud. Combined with #12 in one PR (Closes #11, Closes #12).