Add SF Symbol row icons to sidebar server, channel, and PM rows

[buggerman]

Summary

• Improve at-a-glance scannability of the sidebar by prefixing each row with a small SF Symbol indicating its type. Inspired by Textual's iconography but rendered with native system glyphs so they tint with the system accent and adapt to Liquid Glass automatically.
• Server rows pick up a slashed-out variant when the server is in the stable .disconnected state — same idiom as wifi.slash, microphone.slash. Transient .connecting / .disconnecting states keep the active icon and let the existing dot color carry the signal so the icon doesn't flash mid-handshake.
• No # stripping on channel names; #linux still renders as #linux. The icon is purely additive.

Changes

• Sources/Brygga/Views/ContentView.swift:
  • ServerRow — adds an Image(systemName: serverIconName) between the existing connection-state Circle and the server-name Text. New private serverIconName returns "antenna.radiowaves.left.and.right.slash.circle.fill" when state == .disconnected and "antenna.radiowaves.left.and.right.circle.fill" otherwise.
  • ChannelRow — adds an Image at the start of its HStack. "number.circle.fill" for regular channels, "person.crop.circle.fill" for PMs (channel.isPrivateMessage).
  • All icons render at 13pt with .secondary foreground style — same weight as the existing chevron.down / moon.fill accents in the row, so they sit visually below the colored connection dot and the channel name.

Test plan

• swift build — passes
• swift test — 131 tests pass (no test changes; UI-only)
• swiftformat --lint . — clean
• Manual:
  • Connect to a server. Sidebar shows ● ⊕ Libera style row (state dot + antenna-in-circle icon + name).
  • Disconnect (Cmd+W on the server, or /quit). The icon flips to the slash variant, the dot turns grey.
  • Reconnect. During the brief .connecting window, dot is yellow but the icon stays unslashed.
  • Open a channel #swift — sidebar row shows (number-in-circle) #swift. The # is preserved in the name.
  • PM with ChanServ — row shows (person-in-circle) ChanServ.
  • All three icon shapes share the .circle.fill family so they share weight and cap height.
  • Switch between light and dark mode; verify icons tint correctly.

Risk / rollback

• Pure view-layer change; no model, persistence, or networking touch.
• antenna.radiowaves.left.and.right.slash.circle.fill is shipped in macOS 15 SF Symbols; the deployment floor is already 15.
• Revert: this single commit.