Align sidebar channel icons with the server icon column

[buggerman]

Summary

• The sidebar icons added in Add SF Symbol row icons to sidebar server, channel, and PM rows #32 were 6pt out of vertical alignment: server rows have chevron(12) + spacing(6) = 18pt before their icon, while channel rows had padding(.leading, 12). So the antenna icon sat 6pt to the right of the channels' hash/person icons, making the column read as staggered.
• Bump channel padding to 18 so all icons share a column.

Changes

• Sources/Brygga/Views/ContentView.swift — .padding(.leading, 12) → .padding(.leading, 18) on the per-channel ChannelRow inside the server's ForEach. Comment in source explains the math.

Test plan

• swift build — passes
• swift test — 132 tests pass (no test changes; pure layout)
• swiftformat --lint . — clean
• Manual:
  • Open the app, expand a server. Server's antenna icon, channel hash icons, and PM person icons all line up vertically.

Risk / rollback

• 6pt shift on the channel column. No functional change. Revert: this single commit.