docs: Revise READMEs — correct switching semantics, distinguish from CodexBar

Three substantive changes, applied across all five language READMEs (en,
zh-CN, ja, de, fr):

1. Account switching: replace the previously-claimed "doesn't interrupt
   running Claude Code sessions" with the actually-true positioning.
   The Claude CLI re-reads the keychain on every API call, so an
   in-flight session DOES pick up newly-swapped credentials. What
   CCSwitcher actually delivers is non-destructive switching: each
   account's credentials stay backed up so swapping back doesn't require
   re-OAuth. Section now explicitly calls out the in-flight session
   behavior as Claude CLI's behavior, not CCSwitcher's.

2. Token refresh: previously framed as "inspired by CodexBar." The two
   projects actually take fundamentally different approaches — CodexBar
   POSTs directly to Anthropic's non-public OAuth endpoint with a
   hardcoded client_id extracted from the CLI binary; CCSwitcher
   delegates to `claude auth status` so the official CLI does the
   refresh via its own Anthropic-maintained logic. Rewritten as an
   explicit side-by-side comparison with the trade-offs called out
   (subprocess overhead vs. risk of Anthropic rotating client_id /
   endpoint / adding attestation).

3. Keychain reader: clarified the split. The `/usr/bin/security`
   subprocess approach is specifically for reading Claude Code's
   cross-vendor keychain entry (to avoid the "wants to access keychain"
   prompt). CCSwitcher's own per-account backup store uses native
   `SecItemCopyMatching` directly — no subprocess, no prompt, because
   it's our own keychain entry.

Also corrected: token lifetime "1–2 hours" → "~8 hours" (actual),
dropped "Experimental" qualifier on the keychain reader (production
default for a while), added "In-App Auto-Update" feature bullet
(Sparkle), added section on Team-ID-prefixed App Group (#14), added
section on local JSONL parse cache (#13), fixed broken upstream URL
`lucas-clemente/codexbar` (404) → `steipete/CodexBar`.