fix(desktop): close cross-session leak windows in composer + session refs (#59305)

[JoaoMarcos44]

Summary

Root-cause fix for #59305 ("[Desktop] Chat tab messages leak across sessions — cross-tab content mixing"). Full root-cause writeup in #70609.

Two independent React passive-effect race windows let a fast session switch land content in the wrong chat, plus a third gap where nothing checked that the composer and the session-side refs agreed with each other at the instant of send:

1. use-session-state-cache.ts — activeSessionIdRef / selectedStoredSessionIdRef were mirrored from props via useEffect, which fires one commit after paint. A synchronous read in that window (a stable callback firing against the just-painted new session) still observed the outgoing session's ids.
2. use-composer-draft.ts — the per-thread attachment-scope-swap effect (attachmentScope.$attachments.set(...) on session switch) was the same kind of useEffect. In the race window, $composerAttachments still held the outgoing session's chips — a submit fired in that window would ship them into the new session's turn. This is the actual content leak, not just a visual glitch.
3. submit.ts / session-context-drift.ts — even with 1–2 fixed, the composer (ChatBar) and the session state cache live in separate React subtrees and had no cross-check against each other at send time.

Fix

• use-session-state-cache.ts: session refs now mirror their prop synchronously during render instead of via useEffect. Guarded to fire only when the prop itself actually changed (replicating a useEffect([dep]) dependency array, just synchronously) — an unconditional resync was tried first and would have silently clobbered an intentional imperative ref pin (submit.ts pins a freshly-resumed runtime id into the ref without touching the source atom, by design) on the next unrelated re-render. Caught by a dedicated regression test before it shipped.
• use-composer-draft.ts: the scope-swap effect is now a useLayoutEffect — runs before paint, closing the window. Also added a redacted [composer-rehydrate] diagnostic log (counts/kinds/scope only — never raw refs/URLs/paths) to help triage any future report in this class.
• session-context-drift.ts: new third drift prong comparing the composer's loaded scope (SubmitTextOptions.composerScope, threaded from use-composer-submit.ts) against the submit target. Compared against the target resolved into the same durable lineage-root domain the composer itself uses (resolveComposerSessionKey), not the raw tip id — comparing against the raw tip would have false-positive-aborted every submit into any session that has ever gone through auto-compression. Verified with a dedicated regression test that reproduces the false-positive against the naive comparison before fixing it.
• routes.ts / chat/index.tsx: the primary composer's durable scope key (primaryRouteSelectedSessionId) now prefers the route over a possibly momentarily-stale store selection during a switch.
• chat-runtime.ts: attachmentId() normalizes the value (URL trailing slash, \ vs / paths) before hashing, so a re-attach of the same resource dedupes instead of appearing as a second attachment.

Test plan

• New deterministic regression tests per fix (layout-phase probes, no sleep/timing-dependent repros) — each proven to fail against the pre-fix code, then pass after.
• Regression test specifically proving the lineage-root vs live-tip false-positive is closed.
• Regression test specifically proving the synchronous ref mirror does not clobber an imperative ref pin.
• Full suite across the 17 touched/dependent test files: 286 passed.
• tsc --noEmit: no new errors in any touched file.
• Full project test:ui: no new failures introduced (pre-existing unrelated locale/timing failures confirmed identical via git stash comparison against the same base).

Closes #70609. Related to #59305.

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[teknium1]

Merged via #70986 with your authorship preserved on both fix commits — outstanding work on this one: the #70609 root-cause analysis (two passive-effect race windows + missing submit coherence check) made the salvage mechanical, and all 141 tests + typecheck shipped clean. Only your infographic asset commit was dropped. Fixes #59305 and #70609.