Show per-item translated error messages on /reader operations

[MojRezakhani]

Previously, when secure/unsecure/edit/clear operations failed, errors were only logged to the browser console — the user saw no indication that something went wrong.

Now:

• Each reader item shows its own error inline within its card
• Same alert style as /tagging (icon + text)
• Auto-clears after 5 seconds
• Cleared immediately on retry
• Pattern-matches common errors: wrong password, tag not found, reader unreachable, tag locked, authentication failed
• Paraglide support (EN + DE)

Also fixes a few translation inconsistencies discovered along the way:

• multiple_tags_detected_message: en.json had German text; added missing German version
• reader_initialisation_failure_message: de.json had wrong spelling (with 'z' instead of 's') that didn't match the code; renamed and fixed a typo (Lesegräte -> Lesegeräts)

Root causes fixed:

1. reader.secure/unsecure/edit/clear return { success, message } instead of throwing, so the existing catch blocks never fired
2. Backend returns errors in the 'error' field but feig.ts only read 'message', so error text was lost

[vercel]

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