feat: complete step-by-step invoice creation wizard (#891)

[adenueltech]

Closes #891

What changed

/invoice/new already had a 4-step stepper. This PR fills in what the issue asks for and fixes several bugs in the existing flow.

Validation at each step

• Moved step definitions, help text and validation into src/lib/invoiceWizard.ts (pure, unit tested).
• Fixed: equal-split mode always failed recipient validation, because per-row amounts are empty in that mode. It now validates the total amount instead.
• Fixed: the deadline field is on step 1, but its range was checked on step 3. It is now validated on step 1 (whole number, 1–365).
• Token and recipient addresses use real StrKey checks instead of startsWith/length. Duplicate recipients are rejected, and errors name the row ("Recipient 2 needs…").
• The co-signer threshold must be between 1 and the number of co-signers.
• Errors appear both in the step summary and inline on the field, with aria-describedby pointing at the message (also fixed in FormField).

Navigation

• Submit re-validates every step and jumps to the first invalid one, so a deep link to ?step=4 or a restored draft can't skip checks.
• Pressing Enter on an earlier step advances the wizard instead of submitting.
• The review step has an "Edit" link for each section.

Progress indicator

• Progress bar (role="progressbar") with "Step X of 4" under the existing stepper.

Drafts saved automatically

• The draft snapshot now includes the current step, equal-split total, tags, co-signers, threshold and installments. Restoring brings all of them back and resumes on the saved step. New DraftFormData fields are optional, so existing drafts still load.
• Autosave only starts after the user interacts, so an untouched form no longer shows a "restore draft?" banner on the next visit.
• A "Draft saved HH:MM" status shows when autosave runs.

Help text

• Each step has a one-line description, and every field has helper text (token, deadline, clone deadline, recurring, interval, equal split, total, recipients, tags, co-signers).
• Removed a duplicate DeadlineSuggester that rendered twice on step 1.

How I tested

• npx vitest run src/__tests__/invoiceWizardSteps.test.ts src/lib/offlineDraftDB.test.ts src/hooks/useOfflineDraftAutosave.test.ts: 39 passed
• npx tsc --noEmit: 0 errors
• next lint on the changed files: no warnings or errors

🤖 Generated with Claude Code

[vercel]

@adenueltech is attempting to deploy a commit to the kingsman-99's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@adenueltech Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits