feat(#7): concurrent sidecar session management

[wpm]

Summary

• Rust backend extended with BuildSidecarState (map of session ID → child process), three new Tauri commands (sidecar_start_build, sidecar_stop_build, sidecar_list_builds), and a MAX_CONCURRENT_BUILDS = 3 constant; each build sidecar emits events on session-scoped channels (sidecar://token/<sessionId>, sidecar://complete/<sessionId>)
• New useBuildSessions hook manages concurrent build sessions with startBuild(org, repo), stopBuild(sessionId), and selectSession(sessionId) operations; subscribes to sidecar://build/status events for automatic status updates
• useBuildStream updated to accept an optional sessionId parameter for scoped event channels (falls back to global channels when omitted)
• BuildStreamPanel updated with a session tab bar (shown when multiple sessions are active), backed by SessionStream sub-component that scopes useBuildStream to the selected session
• WaveSidebar updated with ActiveBuildSelector tab bar at the top of the content area (shown when sessions.length > 1 and onSelectSession is provided)
• BuildSession and BuildSessions types added to src/lib/build.ts

Test plan

• All 258 tests pass (pnpm test)
• ESLint passes with --max-warnings 0 (pnpm lint)
• Prettier formatting clean (pnpm format:check)
• Start two builds and verify both sessions appear as tabs in BuildStreamPanel and WaveSidebar
• Switch between session tabs and verify each shows independent output
• Attempting a 4th concurrent build is rejected with "Maximum concurrent builds (3) already reached"
• Stopping a build marks it as cancelled and switches the active session to another running one

Closes #7

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