Recover cleanly from stale thread routes and eliminate root-route preload errors

[redxzeta]

Parent: #125
Priority: P1

Problem

Forkara can restore a persisted thread UUID that no longer has a valid thread-detail snapshot, leaving the URL/thread model inconsistent with the visible workspace.

Hands-on testing reproduced a persisted route like:

/45c1f864-ab00-45fa-a34e-32ad59321dba

while the backend repeatedly reported:

Thread detail snapshot not found for thread 45c1f864-ab00-45fa-a34e-32ad59321dba

The shell could still render a mostly usable new-thread/project surface, so the app looked recovered while the route continued to reference a nonexistent thread.

The same root/restore path also reproducibly emitted a TanStack Router preload exception:

TypeError: Cannot read properties of undefined (reading '_nonReactive')
  at loadRouteMatch(...@tanstack_react-router...)
  at loadMatches(...)
  at RouterCore.preloadRoute(...)

Opening a known-valid thread directly rendered normally, which suggests the primary defect is stale persisted-route recovery rather than general thread rendering.

Goal

Treat a missing/restored thread as a normal recoverable state: clear the stale persisted target, navigate to a clean home/new-thread route, and stop retrying backend detail reads for a thread that does not exist.

Desired behavior

restore persisted thread
        ↓
thread exists? ── yes → render normally
        │
        no
        ↓
clear stale persisted target
        ↓
navigate/replace to clean new-thread or home state
        ↓
no repeated missing-thread RPC stream

Required work

• Trace startup/root-route restoration and persisted latest-thread selection.
• Identify why a missing thread can remain authoritative after backend reconciliation says it does not exist.
• Ensure client-side deleted/missing-thread reconciliation happens before or atomically with route restoration.
• Do not leave a missing UUID in the address bar while rendering an unrelated new-thread surface.
• Investigate the _nonReactive preload failure and determine whether Forkara is asking TanStack Router to preload an invalid/transient route match.
• Fail closed on invalid route targets rather than repeatedly retrying the same missing thread detail snapshot.
• Preserve valid existing-thread deep links and normal resume behavior.

Acceptance criteria

• Starting Forkara with a persisted nonexistent thread id lands on a clean valid route without manual intervention.
• The missing id is removed/replaced from the URL and persisted navigation state.
• Thread detail snapshot not found is not repeatedly streamed for the same stale thread after recovery.
• The root-route recovery path does not emit the TanStack Router _nonReactive preload exception.
• Direct navigation to a valid thread UUID still renders that thread normally.
• Browser/integration coverage includes:
  • valid persisted thread
  • missing persisted thread
  • deleted thread that was previously active
  • root / restore behavior
• Recovery uses route replacement where appropriate so Back does not immediately return to the stale route.

Non-goals

• Do not hide the exception with a broad console filter.
• Do not disable route preloading globally unless proven necessary.
• Do not discard valid thread-resume behavior.

[redxzeta]

Fresh reproduction from the 2026-08-29 read-only Playwright audit on current built-from-scratch (85a9c882c): ordinary navigation still leaves a console exception from TanStack Router preload:

TypeError: Cannot read properties of undefined (reading '_nonReactive')
    at loadRouteMatch
    at loadMatches
    at RouterCore.preloadRoute

The app remained usable, so this is not a crash, but the exception is still present on the current branch and should remain part of this issue's acceptance criteria: normal restore/navigation/preload paths should complete without this console error rather than merely recovering visually.