feat(split): add save/load templates with address-book autocomplete

[JohnOluB]

Closes #155

Status: Draft / blocked

Started implementation and hit two gaps between the issue's assumed
starting state and what's actually in the repo. Opening this early to
surface them rather than guessing at replacement logic.

Blocker 1 — src/lib/stellar/batchSend.ts does not exist

StellarSplit.tsx imports from it:

import { parseCsvRows, validateRows, sendBatch, MAX_BATCH_ROWS } from '@/lib/stellar/batchSend';

src/lib/ currently only contains explorer.ts. This looks like either
a branch mismatch on my end or a file missing from this checkout —
can someone confirm which branch/commit #155 was filed against?

This blocks the template feature directly: loading a saved template
needs to re-validate rows through the same path CSV paste uses, and I
don't want to reimplement validateRows from scratch and risk it
silently diverging from the real validation rules.

Blocker 2 — no existing address-book autocomplete to reuse

Acceptance criteria says the combobox should use "the ARIA pattern
already established in address-book send autocomplete." I checked:

• src/store/contactsStore.tsx exists (useContacts()), but nothing
  in the app currently consumes it
• src/components/StellarSend.tsx (the actual send UI) uses a plain
  text input with a manual paste button — no combobox, no
  autocomplete, no useContacts import
• No role="combobox" / aria-activedescendant usage anywhere in src/
• No headless-UI/combobox library in package.json (Radix, Headless UI,
  downshift, react-aria are all absent)

So there's no existing pattern to reuse yet. Happy to build the first
hand-rolled ARIA combobox for this (matching the plain-Tailwind,
no-dependency style of the rest of the app) — just flagging that it'll
be new, not reused, so review expectations match.

Plan once unblocked

• src/store/splitTemplatesStore.tsx — Context + localStorage,
  mirroring contactsStore.tsx's pattern (done locally, ready to push)
• Save-as-template control on StellarSplit.tsx
• Template list on load (rename / delete / duplicate)
• Per-row combobox sourced from contactsStore
• Export/import as JSON, round-trippable with address-book export format
• Load-template restores exact row set and amounts
• Export → import round-trip test
• Templates survive reload

Questions for maintainers

1. Is batchSend.ts missing from this branch, or was it never committed?
2. Should the combobox be net-new, or is there a WIP branch/PR with the
   address-book autocomplete that I should build on top of instead?

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