fix: keep showing cached usage data on transient errors

[KurtGood]

Problem

When a refresh fails, the popover replaces everything with a full-screen error view — even though the last successful fetch is still in memory (the menu bar icon keeps rendering it). A brief 429 rate limit or a network blip therefore looks like a hard failure:

• The full-screen error hides perfectly good cached data behind an orange triangle.
• All errors get the same treatment: a transient rate limit renders identically to "no credentials configured".

Fix

• Show the full-screen error only when there is no cached data or the error needs user action (unauthorized / session expired / no credentials). Other errors keep the normal detail view and add a compact warning banner at the top: the error text plus a "Showing last fetched data" note (hover shows the full error).
• Track the error kind via a new DataRefreshManager.errorRequiresAuthAction published flag instead of string-matching the localized message (error.contains("Authentication")), which only worked in English and Chinese — as a side effect, the Go to Settings button in the error view now appears for auth errors in every language.
• fetchClaudeOnly no longer clears cached usage data on failure, matching the combined fetchUsage path (previously a failed single-provider refresh wiped the cache; a failed combined refresh kept it).
• New error.showing_cached_data string in all 7 localizations (en, de, fr, ja, ko, zh-Hans, zh-Hant) — non-English translations are machine-assisted, happy to adjust.

Notes

• Popover height accounts for the banner (+53pt) in single- and multi-provider layouts; the Codex column is untouched.
• swift test passes (61 tests).
• Independent of feat: add optional menu-bar reset countdown rings #62.

🤖 Generated with Claude Code