fix(desktop): cache external-session catalog per selection so switching sources shows loaded rows instantly

[liuxiaocs7]

Summary

Switching the import source — or toggling the archived filter / editing the search, and including returning to a source already loaded — blanked the list and flashed the full-width "Reading external conversations…" spinner on every switch, because loadCatalog unconditionally reset the catalog and loading flag with no per-selection cache.

This caches the last loaded CatalogState per (adapterId, includeArchived, search): a cache hit renders the rows immediately and refreshes in the background, while a miss keeps the existing spinner. The background import poll now observes the request generation instead of claiming a new one, so it can never strand an in-flight load's catalogLoading reset.

Fixes #3904

Verification

• tsc for tsconfig.preload.json, tsconfig.main.json, tsconfig.renderer.json — all clean.
• biome check on both changed files — clean.
• node --test dist/main/__tests__/import-tasks-settings-page.test.js — 19/19 pass (15 existing + 4 new source-switching cases).
• Fails without the change: reverting only the component to its pre-fix version makes two of the new tests fail — shows a previously-loaded source instantly with no spinner and does not let a stale background refresh overwrite a newer source selection (17/19); restoring the fix returns them to green (19/19).
• Not run: repo-wide format:check / knip, full npm run build, and the cross-workspace suite. tsconfig.storybook.json reports one pre-existing, unrelated error (UsageRequestLog.sessionName in stories/settings/settings-pages.stories.tsx) that exists on the base commit and this PR does not touch.

Review focus (UI evidence)

I could not capture a headless screenshot of the Electron window in my environment, so the before/after is shown via the deterministic test output above instead of an image. To see it live: run the desktop app, open Settings → Activity → Import Tasks with ≥2 sources, and switch back and forth — a revisited source now appears instantly with no spinner flash (first-time loads still show the spinner).

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code (Claude Opus) — root-cause investigation, the per-selection cache implementation, and the added tests. Reviewed and submitted by the human contributor of record; commit carries a Generated-by: Claude Code trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No