Feedback via MCP and public API; PostHog event loss fixed

What's Changed

• fix(assistant): remove in-app assistant max height by @LysanderKie in https://github.com/langfuse/langfuse/pull/15349
• feat(web): v4 migration entry point — sidebar card + migration side panel by @nmtrang29 in https://github.com/langfuse/langfuse/pull/15086
• feat(MCP, API): Add feedback submission via public API and MCP tool by @LysanderKie in https://github.com/langfuse/langfuse/pull/14923
• fix(search-bar): preset preview clips instead of resizing the search bar (LFE-11067) by @nkabardin in https://github.com/langfuse/langfuse/pull/15377
• feat(nav): promote the Assistant launcher to the mobile top bar (LFE-11067) by @nkabardin in https://github.com/langfuse/langfuse/pull/15378
• chore(storybook): fix two story-guide violations (layout decorator + variant name) by @nkabardin in https://github.com/langfuse/langfuse/pull/15379
• docs: link self-hosting update newsletter from README by @jannikmaierhoefer in https://github.com/langfuse/langfuse/pull/15380
• feat(events): collapse the mobile traces toolbar into a Filters sheet (LFE-11067) by @nkabardin in https://github.com/langfuse/langfuse/pull/15357
• fix(posthog): raise SDK maxQueueSize to stop silent event drops (LFE-14456) by @niklassemmler in https://github.com/langfuse/langfuse/pull/15374
• fix(worker): prevent PostHog export event loss by @sumerman in https://github.com/langfuse/langfuse/pull/15351

Full Changelog: https://github.com/langfuse/langfuse/compare/v4.0.0-rc.0...v4.0.0-rc.1