fix(tui): bound external disk refresh scans

[BatalloLu]

Summary

• Budget MailCache.Refresh() sent-history scans while preserving immediate pending sent-ID delivery checks.
• Make inventory/list status scans more external-disk friendly: read .agent.json once, record slow_fields, and cap MCP identity scans with a truncation marker.
• Guard props/session/detail refresh paths so auto-refresh/Ctrl+R/Ctrl+D do not stack overlapping external-disk scans; detail loads asynchronously and the footer surfaces loading/stale/slow markers.
• Add focused tests for MailCache sent-scan throttling, inventory slow/truncated markers, and props in-flight/detail behavior.

Context

This PR targets TUI lag observed on large .lingtai networks mounted on an external disk. It does not claim to eliminate every possible exFAT/external-volume slowdown, but it removes several every-second full-scan and synchronous-detail paths that matched the observed bottlenecks.

Validation

• git diff --check
• go test ./internal/fs ./internal/inventory -count=1
• go test ./internal/tui -run 'TestProps(CtrlR|CtrlD|ReloadStatusHint|AutoReloadCmd|LoadDetail)|TestAppAutoRefresh(SkipsPropsLoadWhenInFlight|TickKeepsKanbanDetailLive|TickReloadsAndRearms)|TestAutoRefreshActiveViewOnlyReloadsKanban' -count=1
• go test ./internal/tui -run '^$' -count=1 (compile-only)

Not covered / follow-up

• Full go test ./... was intentionally not run because existing OAuth tests may open a real browser.
• No live external-disk Wenda validation was performed in this PR.
• No installed binary, hotpatch, release, installed wheel/venv, or Wenda runtime/mailbox/SCU/P0 state was changed.