Add locate-floor action for author-only topics

[BugenZhao]

Summary

• add a Swift-side TopicPostLocator that caches full-topic post locations and scans full-topic pages on demand when author-only needs to resolve a post back to its real floor
• wire Locate This Floor through topic details and reply-chain navigation so author-only can dismiss back to the originating topic and jump to the matching full-topic post
• keep author-only floor display unchanged while seeding locator state from loaded full-topic replies only

Testing

• make swiftformat
• make build
• AXe manual verification on mnga://topic/45315204
  • enter This Author Only
  • open author-only reply menu and tap Locate This Floor
  • confirm it returns to the original topic and lands on the post's real full-topic floor

[Copilot]

Pull request overview

Adds a Swift-side mechanism to “locate” an author-only reply’s corresponding floor in the full topic, enabling navigation back to the originating topic and scrolling/loading the correct page when needed.

Changes:

• Introduces TopicPostLocator to cache post locations and scan topic pages on demand to resolve a post’s full-topic floor/page.
• Wires a Locate This Floor action through topic details, author-only topic navigation, and reply-chain navigation.
• Seeds locator state from loaded full-topic replies (while keeping author-only floor display unchanged).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
app/Shared/Views/TopicDetailsView.swift	Adds locator state + callback plumbing; seeds locator from topic responses; adds locate/jump helpers and navigation wiring.
app/Shared/Views/PostReplyChainView.swift	Passes locate callback through author-only navigation destination.
app/Shared/Models/TopicPostLocator.swift	New model to cache/scan full-topic pages to resolve a post’s page/floor location.

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