✨ add UserAttentionService to gate background pollers on window visibility

[guiyanakuang]

Closes #4405

Summary

• New UserAttentionService (commonMain) exposes per-surface visibility as StateFlow<Boolean> — main window and search window. Comes with two helpers: attentionOn(vararg AttentionSurface) to combine surfaces, and CoroutineScope.launchWhileAttentive(...) to drive polling loops gated on attention.
• Desktop binds DesktopUserAttentionService (wraps DesktopAppWindowManager); headless binds HeadlessUserAttentionService (permanently false).
• DesktopNetworkProfileService is the first migration: the 60s Windows firewall scan now only runs while the main window is visible. On window-show the helper fires runDetection() immediately, so the user sees fresh diagnosis the moment they open the window — not after waiting a poll interval.

Design notes

• Opt-in per service. SyncPollingManager / CleanScheduler are not migrated — they need to run regardless of UI state. The abstraction is here for the pollers that only matter when a user can see the result.
• Per-surface granularity. Future detectors can subscribe to MAIN_WINDOW only, SEARCH_WINDOW only, or both via attentionOn(MAIN_WINDOW, SEARCH_WINDOW). Network detection only renders warnings in the main window so it gates on MAIN_WINDOW.
• Bubble window is not exposed. No consumer needs it; add when one does.
• WindowInfo.show is the app's logical show state, so minimized windows still count as visible. If we later decide minimized should suspend polling, change the mapping in DesktopUserAttentionService only — consumers don't need to change.
• Headless wins a small efficiency for free: the always-false flows mean runDetection() is never invoked in headless mode, without explicit if (headless) checks scattered through services.
• runOnResume = true is the default so a user opening the main window sees a fresh diagnosis immediately, not after a 60s wait.

Files

• app/src/commonMain/kotlin/com/crosspaste/app/UserAttentionService.kt — new
• app/src/desktopMain/kotlin/com/crosspaste/app/DesktopUserAttentionService.kt — new
• app/src/desktopMain/kotlin/com/crosspaste/headless/HeadlessUserAttentionService.kt — new
• app/src/desktopMain/kotlin/com/crosspaste/DesktopUiModule.kt — bind UserAttentionService to desktop impl
• app/src/desktopMain/kotlin/com/crosspaste/headless/HeadlessModule.kt — bind to headless impl
• app/src/desktopMain/kotlin/com/crosspaste/DesktopNetworkModule.kt — pass new dep into DesktopNetworkProfileService
• app/src/desktopMain/kotlin/com/crosspaste/net/DesktopNetworkProfileService.kt — migrate to launchWhileAttentive

Test plan

• ./gradlew app:desktopTest — all desktop tests pass.
• Manual on Windows: open main window → confirm firewall check fires once immediately, then every 60s; close window → confirm no further runDetection log lines until window reopens.
• Manual on Windows headless install: confirm no Windows firewall COM calls happen at all (no "queryProfile" / "queryMDnsAllowed" log lines).
• Visually confirm that on first launch (main window auto-shows), the diagnosis & menu-bar warning entry populate within ~3s of startup as before.