Refactor existing modes into generator registry

[t1]

Extract curl and HTTPie generation from the form submit handler in app.js into standalone generator functions. Introduce a generator registry (Map<String, Function>) where each key is the display label and each value is a generator function with signature generate({ method, url, headers, body, contentType, schema }) → string.

The form submit handler should call generators[label](params) for all copy-format modes and copy the result to clipboard. The Try mode remains special (sends the actual request).

No new formats, no UX changes. The 3-segment toggle stays as-is.

Part of #11. See spec: docs/superpowers/specs/2026-04-12-issue-11-output-formats-design.md

[t1]

Agent: starting work

Type: Refactoring (pure code structure change, no new features)
Assessment: Current curl and HTTPie generation is embedded in form submit handler. This refactoring extracts them into standalone generator functions in a registry pattern.
Approach:

• Create generator registry Map in app.js with curl and HTTPie generators
• Each generator: ({ method, url, headers, body, contentType, schema }) → string
• Update form submit handler to call generators[label](params) for copy modes
• Try mode remains special (sends actual request)
• No UX changes, no new formats

Files: core/src/main/resources/app.js, corresponding browser tests

[t1]

Agent: done

Commit: ff6e48a
Summary: Extracted curl and HTTPie generation logic into standalone generator functions within a pluggable registry pattern. The form submit handler now calls generators[mode](params) for copy modes, making it easy to add new output formats in future issues. No behavior change — all existing browser tests pass.
Tests: All 334 BrowserTest tests pass, including existing curl/httpie clipboard tests
CI: passed