refactor: extract window session protocol

[PathGao]

Summary

• move persisted window-state cleanup and saving into the windowSession module
• move detached-tab staging, acknowledgement, cancellation, and error handling into the same module
• keep MarkdownViewer responsible only for tab-specific policy and callbacks
• relocate structural regression checks with their implementation

Depends on #271 and should be merged after it. This is the second focused step in the #236 componentization sequence; listener ownership will follow separately.

Validation

• npm run check
• npm test (117 passing)
• cargo test (18 passing)

[PathGao]

@alecdotdev This is the next dependent componentization PR after #271. It isolates the window-state persistence and detached-tab transfer protocol; listener ownership remains a separate follow-up to keep review scope small.

[alecdotdev]

@PathGao minor conflict emerged after merging #271 other than that, LGTM

[PathGao]

@alecdotdev #271 is now merged, so I rebased this dependent branch onto current master and reran its parent relationship. #273–#275 were rebased linearly on top of the rewritten chain.

[PathGao]

All checks are now green for #272 through #278. The remaining #279/#280 build matrix is still running. Once #272 merges, I will rebase and re-verify the next branch in the stack immediately.