refactor: change analog/digital field to a more generic mode field

[pskillen]

Problem

Channels currently use a coarse mode field (analogue | digital | other) derived from vendor CPS "Channel Type". That is enough for OpenGD77 today (FM vs DMR) but does not express the modes we want to support across vendors:

Analog: FM, AM, SSB (USB and LSB)

Digital: DMR, YSF, Tetra

Issue #11 already calls for mode pills and filters (Digital / FM / AM / SSB) in the channel table; the data model needs to catch up before we can implement those consistently.

Intended outcome

Replace the binary analogue/digital abstraction with a specific channel mode value that drives UI pills, filters, and conditional form fields.

Documentation

Create docs/reference/channel-modes.md as the canonical reference:

• List every supported mode value (FM, AM, SSB-USB, SSB-LSB, DMR, YSF, Tetra, …)
• Static colour mapping per mode (hex + Mantine token), consistent with band pill styling
• Notes on which form fields apply per mode (e.g. hide CTCSS/DCS on digital modes)

Data model

• Update ChannelMode (or equivalent) in src/models/codeplug.ts
• Update docs/features/data-model/ to match
• Migration / import mapping: OpenGD77 Channel Type → new mode values (Analogue → FM, Digital → DMR, etc.)
• Preserve backward compatibility for existing projects in localStorage (sensible defaults for other)

UI

Update channels list, detail, and edit pages:

• Mode pill with documented colours
• Edit form mode selector with the full enum
• Hide/show fields based on mode (tones, colour code, timeslot, contact/TG list, etc.)
• List filters for mode (per feat: CRUD channels #11)

Implementation should mirror the band reference pattern: src/lib/channelModes.ts (or similar) must match docs/reference/channel-modes.md.

Affected

• src/models/codeplug.ts — ChannelMode type and defaults
• src/lib/import/ — OpenGD77 channel type mapping
• src/routes/channels/ — list, detail, edit
• docs/reference/channel-modes.md (new)
• docs/features/data-model/

Notes / dependencies

• Unblocks mode pills and mode filters in feat: CRUD channels #11 (CRUD channels).
• Related to feat: CRUD channels #11 mention of "Highlight digital/fm/am/ssb channels".
• Does not cover multi-mode channels (single entity carrying both analog and digital) — see the separate multi-mode channels issue.

Out of scope

• Multi-mode / dual-mode channel modelling (analog+digital as one logical channel).
• Export-layer mode mapping for every vendor (initial focus: internal model + OpenGD77 import).

Workflow note

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