fix(session): correct shell-history copy and load command history off-main

[crmolinaz]

Follow-up to the per-tab shell history / session restore work, addressing review feedback on upstream PR manaflow-ai#6548.

Changes

Correct the "grouped by project" copy (history is keyed by surface ID).
Per-tab shell history is keyed by the tab's stable surface UUID, not by project directory. The shipped localized strings (xcstrings + all web locales) were already accurate, but three source references still described the old behavior. Aligned them with what actually happens:

• SessionSection.swift — section doc comment and the String(localized:) defaultValue fallback (the fallback had drifted out of sync with the real translation).
• SessionCatalogSection.swift — the persistShellHistory key doc comment.

Load command history off the main thread.
Opening the command history from the palette decoded the per-surface JSON synchronously on @MainActor, which can block the UI for a tab near the entry cap. showCommandHistoryForFocusedSurface now loads and decodes on a detached task and presents the window back on the main actor.

Notes

A third review item — the unconditional declaration of the debugRestoreSessionConfirmationHandler test seam — was intentionally left as-is: its usage is already #if DEBUG-guarded, and the unguarded declaration matches the two pre-existing sibling seams next to it. Guarding only this one would make it inconsistent with the established convention.

Testing

• Localization audit: no remaining "grouped by project" / "namespaced by project directory" wording in any locale; no new user-facing strings introduced.
• Tagged Debug build compiles cleanly.