MainBar: refresh sample rate list when channels popup closes

[heeen]

Summary

• Connect channels popup closed() signal to on_config_changed() to refresh the sample rate dropdown after toggling channels

Problem

Some drivers change available sample rates based on enabled channel count (e.g. sipeed-slogic-analyzer: 200MHz@16ch, 400MHz@8ch, 800MHz@4ch). The sample rate dropdown was not refreshed after toggling channels, so users had to manually change the rate to trigger a re-query.

Fix

When the channels popup closes, call on_config_changed() which calls commit_sample_rate() → update_sample_rate_selector(), refreshing the rate dropdown.

Test plan

• Open PulseView with a device that has channel-dependent sample rates
• Toggle channels in the channels popup
• Close the popup and verify the sample rate dropdown updates immediately

[vk4tmz]

Tested this PR successfully with a Hantek 1008C driver whose available samplerates depend on the enabled-channel count.

Closing the Channels popup refreshes the list correctly:

• CH1: 2.4 MSa/s and 800 kSa/s
• CH1+CH2: 1.2 MSa/s and 400 kSa/s
• 3–4 channels: 600 kSa/s and 200 kSa/s
• 5–6 channels: 400 kSa/s and 133.333 kSa/s
• 7–8 channels: 300 kSa/s and 100 kSa/s

I found one remaining case involving restored settings. If PulseView exits with CH1+CH2 enabled, after restarting both channels are restored but the samplerate list initially contains the CH1-only rates. Opening and closing the Channels popup refreshes it correctly.

It appears the initial samplerate list is queried before the saved channel-enable states are restored, while the closed() signal only covers interactive use of the popup. A deferred refresh after session restoration completes may also be needed.

Built successfully against PulseView master af021987 with Qt 5.15.13 and libsigrok 0.6.0-git.