feat: show mode-appropriate fields on channel detail and edit pages

[pskillen]

Problem

#11 added channel CRUD with list, detail, and edit pages, but most fields are visible regardless of mode. The edit form partially gates fields today (isAnalogue → tones, isDigital → DMR block), yet many RF fields and the detail page still show everything — including a permanent DMR section and analogue tone rows on digital channels.

#45 will extend ChannelMode beyond analogue / digital / other (FM, AM, SSB-USB, SSB-LSB, DMR, YSF, Tetra, …). Without a documented field→mode matrix and consistent UI gating, each new mode will require ad-hoc conditionals.

Intended outcome

Documentation

Document which channel fields apply to which modes. Extend (or companion to) docs/reference/channel-modes.md from #45 with a field applicability table, e.g.:

Field / section	FM	AM	SSB	DMR	YSF	Tetra	…
RX/TX tone	✓	✓	✗	✗	✗	✗	…
Colour code	✗	✗	✗	✓	…	…	…
Contact / TG list	✗	✗	✗	✓	…	…	…
…

Include shared fields (frequencies, bandwidth, power, squelch, location, scan/APRS) and mode-specific ones. Implementation helper (src/lib/channelFieldVisibility.ts or similar) should mirror this doc.

Channel edit page

• Show only fields applicable to the selected mode.
• Hide empty sections entirely (no "DMR" block on an FM channel).
• When mode changes, decide whether to clear now-irrelevant values or preserve them silently for mode switches back (document the choice).

Channel detail page

• Same gating: omit sections/rows that do not apply to the channel's mode.
• Today detail.tsx always renders RF tones and a full DMR section regardless of channel.mode.

Affected

• docs/reference/channel-modes.md (from refactor: change analog/digital field to a more generic mode field #45) — field applicability section
• src/routes/channels/edit.tsx — conditional form fields
• src/routes/channels/detail.tsx — conditional detail sections
• Possibly a small shared visibility helper used by both pages

Notes / dependencies

• Depends on refactor: change analog/digital field to a more generic mode field #45 for the expanded mode enum and colour mapping doc.
• Builds on feat: CRUD channels #11 (CRUD UI already exists; this is polish/consistency).
• Edit form already has partial gating (isAnalogue / isDigital); extend and align with the full mode set.

Out of scope

• Multi-mode channels (single entity with FM + DMR) — separate issue (feat: support for multi-mode channels #46).
• Hiding list-table columns by mode (could follow later).

Workflow note

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