refactor(new-proposal): remove unreachable SubmitStep dead code

[sidhujag]

Summary

After the Codex PR8 round 5 P2 refactor, onPrepare redirects
straight to /governance/proposal/:id — the canonical, reload-safe
status page that already hosts the Pay-with-Pali / syscoin-cli /
OP_RETURN lanes and the attach-collateral form. That made the
in-wizard SubmitStep unreachable: setPrepared(envelope) is
followed immediately by history.replace(nextPath), so
currentStep === 'submit' never renders, and a regression test
already asserts wizard-panel-submit is null post-prepare.

This PR deletes that dead code.

Removed

• SubmitStep component and its 'submit' entry in STEPS /
  STEP_LABELS.
• currentStep === 'submit' render branch.
• Toolbar currentStep !== 'submit' guards (always true now).
• onAttachCollateral handler — the attach-collateral flow lives on
  the status page (ProposalStatus.js) which has its own handler.
• txidInput / attaching / attachError state — only consumed by
  SubmitStep.
• fmtSys local helper — only used inside SubmitStep.
• Imports no longer referenced: PayWithPaliPanel, usePaliAvailable,
  COLLATERAL_FEE_SATS, HEX64_RE.
• Doc-header "Four steps: Basics, Payment, Review, Submit" rewritten
  to describe the three reachable steps plus the post-prepare
  redirect.

Kept intentionally

• prepared state + setPrepared(envelope) — still participates in
  the dirty-guard dirty = !formsEqual(form, baseline) && prepared == null
  that suppresses the unsaved-changes modal during the prepare →
  redirect handoff. Removing it would touch the leave-guard logic
  and is out of scope here.
• The "why redirect instead of in-wizard submit" comment block on
  onPrepare — historical rationale is still useful context for the
  pre-authorised allowedPathRef navigation.
• The regression-guard test assertion that wizard-panel-submit
  never mounts post-prepare — now trivially true, but serves as a
  trip-wire if anyone re-adds an in-wizard submit panel without the
  redirect.

PayWithPaliPanel (and usePaliAvailable) remain in the codebase:
still consumed by ProposalStatus.js.

Test plan

• npm test -- --testPathPattern='pages/NewProposal' — 30 passed
• npm test -- --testPathPattern='(ProposalStatus|PayWithPaliPanel)' — 39 passed (sanity, unaffected by this change)
• Manual smoke: draft → Basics → Payment → Review → Prepare →
  should land on /governance/proposal/:id with no unsaved-changes
  modal flash; Back / Save-draft render correctly on every reachable
  step.

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