fix(lora): update LoRa preset settings to reflect accurate BW, SF, and CR values; remove frequency from display

[Jeymz]

Summary

fixes #189

The Relay Airtime Share caption assumed SF 8 and displayed a frequency that has no effect on the calculation.

• Default LoRa preset is now BW 62.5 kHz / SF 7 / CR 4/5 (was SF 8). This matches the US preset already used in public/home.js.
• The "Assumed LoRa preset" caption now shows BW, SF, CR and preamble only. The frequency is no longer displayed.

Changes

• cmd/server/relay_airtime_share.go: defaultLoRaPreset() uses SF 7 and has no default frequency. Comments updated.
• cmd/server/config.go: doc comment for AnalyticsConfig.LoRaPreset updated.
• config.example.json: loraPreset now has "sf": 7 and no freq. The _comment_ is updated.
• public/analytics.js: removed the frequency from the caption in renderRelayAirtimeDumbbell. The function is also exposed on window for testing, like the other _analytics* exports.

Compatibility

• The freq config field is still accepted, and preset.freq_hz is still returned by the API. Nothing is removed. Dropping it from the API would be a breaking change, so it can be deprecated in a later release.
• Deployments that copied the old config.example.json have "sf": 8 set explicitly. Their display and shares will not change until they edit analytics.loraPreset.sf. This is worth a release-notes line.

Effect on results

SF changes symbol time (SF 7 is about half of SF 8). The preamble is 32 symbols for both, so it is unchanged. Shares are normalized, so percentages shift slightly. For example, the ADVERT/ACK test case moves from 83.48% / 16.52% to 83.88% / 16.12%.

Tests

• relay_airtime_share_test.go: the hand-computed ToA shares and the derivation comment are redone for SF 7. The preset response check no longer requires a non-zero frequency.
• tests/unit/test-frontend-helpers.js: new regression test that the caption shows BW 62.5 kHz, SF 7 and CR 4/5 and never contains MHz.

Results:

• cd cmd/server && go test ./...: pass
• cd internal/lora && go test ./...: pass
• node tests/unit/test-frontend-helpers.js: 728 passed, 0 failed

Not yet done: browser validation of the Analytics page (AGENTS.md rule 2).

Performance

No hot-path changes. This is a default value change plus the removal of one string from a caption.

Follow-ups (not in this PR)

• Derive the preset from observer radio data instead of a global config default.
• Deprecate freq / freq_hz in the API.