fix: use cache: no-store on RPC transports to prevent stale block hashes

[drappi-ai]

Summary

• Adds fetchOptions: { cache: "no-store" } to the http() transports in both config.ts (wagmi config) and providers.tsx (seismic-react public transport)
• Also includes the no-bracket warning UX from Show bracket-required warning in group join/create flows #210

The cache: "no-store" option tells the browser's fetch() API to bypass any HTTP caching layer for RPC calls. Users reported stale recentBlockHash values (minutes to hours old) in shielded transactions. While the root cause is being addressed in seismic-viem (SeismicSystems/seismic#123), this is an additional belt-and-suspenders fix on the dapp side.

Test plan

• Shielded writes (submit/update bracket) still work
• Signed reads (load bracket before deadline) still work
• No-bracket warning shows in group join/create flows when wallet has no entry

[cdrappi]

Probably just want to fix this in seismic-viem