Tighten desktop navigation and channel details sizing #210

Agentation feedback from the running 1100×760 desktop app:

• Issues and PR navigation icons look too large beside Home and Channels.
• The desktop-tools control needs space before the new-tab “+” button.
• The Channel details summary is too large and can occupy too many lines.
• All Channel details typography should match the channel tab labels.

Reduce the two navigation icons to 16px, add an 8px gap after channel access controls, use the tabs' 12px text size throughout details, and clamp the summary at four lines. Verify in the existing native desktop channel with a long summary.

Part of #5.