fix: emoji/UTF-8 listing validation (#506) and stale price caches after update (#507)

[Hey-Yetunde]

Summary

Fixes two reported bugs:

#506 — create_prompt fails when preview/title contains emoji

Root cause: The frontend validated title/preview using JS .length (UTF-16 code units) while the on-chain PromptHash contract enforces its MAX_*_LEN limits in UTF-8 bytes (soroban_sdk::String::len()). An emoji is 2 UTF-16 units but 4 UTF-8 bytes, so a title/preview that "fits" the client cap was still rejected on-chain with InvalidFieldLength.

Fix:

• src/lib/validation/listing.ts: added exported utf8Length() (UTF-8 byte length) and switched imageUrl, title, category, and previewText validation to it, with byte-accurate error messages — the client now enforces exactly what the contract enforces.
• src/pages/sell/CreatePromptForm.tsx: title/preview live counters now show UTF-8 byte usage so users see the true remaining budget.
• contracts/prompt-hash/src/contract.rs: documented the UTF-8 byte semantics of validate_len and the matching client-side check.
• Tests: src/test/listingValidation.test.ts (5 tests) covering emoji within/over the byte limits.

#507 — stale React Query cache after price update

Root cause: After update_prompt_price, the invalidation helpers only invalidated ["marketplace-prompts"], ["created-prompts"], ["purchased-prompts"], ["saved-prompts"], ["prompt-access"]. The prompt detail view (["prompt-detail", id] — browse modal and /prompt/:id) and the cart price snapshot (["marketplace-prompts-cache"]) read price from separate keys that were never invalidated, so they kept showing the old price until a manual refresh.

Fix:

• src/hooks/useContractSync.ts: invalidateAllPromptQueries now also invalidates ["prompt-detail"] (prefix match covers every id) and ["marketplace-prompts-cache"].
• src/pages/sell/MyPrompts.tsx: refreshPromptLists invalidates the same two keys after a price update.
• src/hooks/useContractSync.test.ts: updated for the two new keys (7 invalidations).

Verification

• yarn vitest run: new + updated suites green — useContractSync.test.ts (8), listing.test.ts, listingValidation.test.ts (5).
• No new failures in the full suite vs main (the remaining failures — dashboard/checkout/PromptCard/API unlock/etc. — are pre-existing on main).
• eslint on changed files: 0 errors.
• tsc -b: no errors in changed files (the repo's remaining typecheck errors are pre-existing on main).

Note: contracts/prompt-hash/src/test.rs and the contract lib have pre-existing compile breakage on main unrelated to these issues.

Closes #506
Closes #507

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