Refresh the README with the latest changes

[Sythos]

Brings the root README.md up to date with what landed since v0.1.16 (the issue-specific parts, like the cleartext warning, the reconnect back-off, the mute bar and the link rules, were already described by their own pull requests):

• Local files: the per-platform config, data and log folders, the migration from ~/.cordiale, atomic and private writes, the .corrupt quarantine and the 1 MiB log with one rotated copy.
• TLS trust: REST and the socket verify against the operating system's store, and a rejected certificate stops the session with a message.
• Chat: the 5,000-row cap per window and the in-place direct-message append.
• Packages and CI: the Linux packages declare their runtime libraries and CI checks them, and CI now builds macOS and the ctap-hid feature (mentioned next to the passkey notes).
• Workspace: the module layout of cordiale-ui after the split of main.rs, packaging/check-linux-deps.sh and the workflows.
• Known gaps: TLS against a private CA, the file migration on real Windows and macOS machines and the history cap in a long-running window are listed as not yet tried against a live server.

Only README.md changes.

Branch: docs-readme-refresh