Detect user-list double-click manually instead of via SwiftUI gestures

[buggerman]

Summary

• Single-click on a nick in the right-side user list was still flaky after Fix flaky single-click selection in the user list #22 — simultaneousGesture(TapGesture(count: 2)) didn't fully resolve the click-deferral.
• Root cause: even with simultaneousGesture, SwiftUI's gesture resolver still has to wait the OS double-click window before committing a count=1 tap, because the count=2 recognizer is a competing alternative. On busy channels every MODE / AWAY update re-renders rows, and a row that re-renders mid-deferral loses the gesture state — so the click silently drops.
• Fix: drop SwiftUI's count=2 gesture entirely. Attach only an onTapGesture (single-tap, no deferral) and detect double-clicks manually by checking whether the same user was tapped within NSEvent.doubleClickInterval. Mirrors what NSTableView does natively.

Changes

• Sources/Brygga/Views/ContentView.swift — UserListView:
  • Adds @State private var lastClickedID: User.ID? and @State private var lastClickedAt: Date?.
  • Replaces .simultaneousGesture(TapGesture(count: 2)...) with .onTapGesture { handleClick(on: user) }.
  • New private handleClick(on:) that selects on first click, opens PM on a second click within NSEvent.doubleClickInterval on the same row, and clears the double-click state on a successful PM open or channel switch.
  • .onChange(of: channel.id) now also resets the double-click tracker so a click in one channel can't accidentally count as the first half of a double-click after switching channels.

Test plan

• swift build — passes
• swift test — 131 tests pass
• swiftformat --lint . — clean
• Manual:
  • Single-click any nick — row highlights immediately, every time, even on a busy channel with frequent MODE / AWAY traffic.
  • Double-click a nick — opens / focuses the PM tab.
  • Click nick A, wait ~1 s, click nick B — both selections register cleanly; no PM opens.
  • Click nick A, then within ~250 ms click nick A again — PM opens.
  • Right-click — context menu opens regardless of selection.
  • Switch channels mid-double-click sequence — the half-click doesn't carry over.
  • Verify NSEvent.doubleClickInterval honors a non-default System Settings → Accessibility → Pointer Control double-click speed.

Risk / rollback

• Pure view-local logic change. No model or persistence touch.
• Worst case: a user with an extremely fast manual double-click cadence (< system interval) could see two single-click selections instead of a PM. Easy follow-up: clamp the interval to a minimum of e.g. 100 ms.
• Revert: this single commit.