Fix dead Hero CTAs, fake landing stats, wrong chain claims, getUser validation

[codeX-james]

Summary

• Hero.tsx: "Start Streaming" now routes to /dashboard via a Next Link; removed the dead "Governance App" button since there's no governance UI in the repo.
• Stats.tsx: replaced fabricated $240M+ TVL / 85k+ streams / $4.2M savings with honest placeholder copy (network/status/settlement info) instead of made-up numbers.
• FAQ.tsx: corrected the "Which chains do you support?" answer — FlowFi is Stellar/Soroban, not Ethereum/Arbitrum/Optimism/Polygon/Base.
• user.controller.ts: getUser now validates publicKey the same way getUserEvents already does — missing/non-string returns 400, malformed Stellar key returns 400 — instead of silently passing garbage to Prisma.

Test plan

• npx eslint clean on the three changed frontend components
• Updated backend/tests/user.controller.test.ts getUser suite: added 400 cases for missing/malformed publicKey, updated existing 404/200 cases to use valid-format Stellar keys
• Manually verified the validation regex/control-flow matches the already-tested getUserEvents pattern
• Full vitest suite could not be run end-to-end: backend/src/services/sse.service.ts currently fails to parse on main (pre-existing, unrelated to this PR — a class method signature appears to have been dropped in a prior merge, e.g. addClient(...) is missing along with isShuttingDown/checkCapacity/getActiveIpCount). This already breaks Backend CI on main independently of this change. Flagging separately rather than guessing at the missing implementation in this PR.

Closes #615
Closes #614
Closes #612
Closes #538