fix: show stored tickers when token info is unavailable

[erubboli]

Summary

• Favourites now render the ticker stored in the user's preference when the on-chain token info lookup fails (e.g. wallet daemon unreachable) instead of a bare ??? placeholder.
• /api/prefs hardening while favourites became user-visible output: reject tickers containing control or bidi-override characters; legacy plain-string entries are normalized to the {tokenId, ticker} object shape on both read and write.
• Re-pinning an unresolvable token stores a short-id placeholder instead of the literal ???.

Test plan

• tsc --noEmit clean; 877 tests green (74 files) incl. new daemon-down fallback test and bidi/control ticker rejection; astro build OK.
• Independent security review: 0 blockers, 0 majors; 4 minors all addressed in this PR.