fix(desktop): route notification clicks to thread context

[tlongwell-block]

Summary

• carry threadRootId through desktop notification targets and existing sprout://message deep link navigation
• add threadRootId to channel route search params and fetch it with the target event for route backfill
• merge cached target + fetched target/root route events into the channel timeline so reply notification clicks can open thread context without dropping cache-only mock/feed hits

Testing

• pnpm --dir desktop typecheck
• pnpm --dir desktop exec biome check src/app/AppShell.tsx src/app/navigation/resolveSearchHitDestination.ts src/app/navigation/useAppNavigation.ts src/app/routes/ChannelRouteScreen.tsx 'src/app/routes/channels.$channelId.posts.$postId.tsx' 'src/app/routes/channels.$channelId.tsx' src/features/channels/ui/ChannelScreen.tsx src/features/notifications/lib/desktop.ts src/features/notifications/use-feed-desktop-notifications.ts src/shared/api/types.ts src/shared/ui/markdown.tsx src/shared/useMessageDeepLinks.ts
• pnpm --dir desktop test
• pnpm --dir desktop exec playwright test tests/e2e/profile.spec.ts -g "notification settings drive" --project=integration
• Manual/temporary Playwright 4-deep notification gate: seeded root → reply depth 1 → depth 2 → depth 3 → depth 4 target, clicked a mock desktop notification carrying target + root, verified #engineering opens the message thread panel and contains the depth-4 target. Result: root-only route fetch is enough because existing useLoadMissingAncestors backfills intermediate ancestors; no new ancestor walker needed in this patch.

Notes

Pre-push hook was bypassed because the repo's broader desktop-tauri-test / desktop-tauri-clippy hook fails in unrelated Rust code on this machine/toolchain with use of unstable library feature round_char_boundary in desktop/src-tauri/src/commands/agent_discovery.rs.

@dawn please review the route backfill/cache-preservation shape.

[wesbillman]

@codex please review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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