fix: prices lookup cooldown, factory warning, bounded responses, dev-stack docs

[gboigwe]

Closes #486
Closes #487
Closes #488
Closes #489

Changes

• [enhancement] No rate limiting on prices page - unlimited backend API calls #486: the prices page ignores repeat submissions while a lookup is in flight or within a 2s cooldown, and disables the Look up button during the cooldown.
• [bug] sorobanService.initialize() called unconditionally - fails if factory contract not deployed #487: sorobanService is already initialized lazily (not at import). initialize() now returns early when no factory contract is configured, and the Factory contract not initialized warning is logged once instead of on every load.
• [enhancement] No Content-Length header validation on RPC responses #488: added src/lib/safe-fetch.ts with readJsonWithLimit. It rejects when the declared Content-Length is over 5 MiB and also counts the streamed body, so a missing or understated header is still caught. It is used for the Horizon account fetch, the leaderboard API and backend requests, and an oversized backend response now surfaces as an error instead of an empty body. Unit tests cover the reader.
• [enhancement] dev-stack.sh script not documented #489: added docs/dev-stack.md (what scripts/dev-stack.sh starts, when to use it, prerequisites, what each run does, files, troubleshooting) and linked it from the README.

Scope notes

• Stellar RPC calls made through the Stellar SDK are not covered by [enhancement] No Content-Length header validation on RPC responses #488, because the SDK owns those requests.
• Not built or run locally; CI on this PR is the first run.

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