feat: wire public-visibility uploads through the external-signer flow

[Nic-dorman]

⚠️ Blocked on WithAutonomi/ant-client#39

This branch does not compile against today's ant-core pin. It calls Client::file_prepare_upload_with_visibility and reads FileUploadResult.data_map_address, both of which land with ant-client #39 (approved by @mickvandijke, waiting to merge). The plan once #39 lands:

1. Bump the ant-core rev in src-tauri/Cargo.toml to the post-merge commit
2. `cargo check` — verify the symbols resolve
3. Flip this PR out of draft

Opened as draft now so the ant-gui work is discoverable alongside the upstream PR.

Summary

Re-enables the Public visibility option in the upload dialog (currently disabled with "Coming soon") by threading a visibility flag through the external-signer flow. Public uploads yield a single shareable on-network chunk address (the published DataMap); clicking it in the uploads table copies it to the clipboard for sharing.

• Why not `data_map_store` after `finalize_upload`? That path hard-requires `require_wallet()?`; ant-gui has no Rust-side wallet — all signing goes via WalletConnect on the frontend. `store_paid_chunks` and friends are `pub(crate)`, so hand-rolling the storage flow isn't an option either. Upstream EVM Network Naming from Connect Wallet/MetaMask #39 bundles the DataMap chunk into the existing one-signature payment batch, which is what lets ant-gui thread a `visibility` flag through its existing code path.

Changes

Backend (`src-tauri`)

• `autonomi_ops.rs` — imports `ant_core::data::Visibility`, threads `StartUploadRequest.visibility` (serde default → Private) into `file_prepare_upload_with_visibility`, extracts `FileUploadResult.data_map_address` into `UploadResult.public_address` on both `confirm_upload` and `confirm_upload_merkle`.
• `config.rs` — `UploadHistoryEntry.public_address: Option` persisted to `upload_history.json`.

Frontend

• `stores/files.ts` — `FileEntry.public_address`, `UploadHistoryEntry.public_address`, `getUploadQuote(path, visibility)`, passes visibility into `start_upload`, captures `result.public_address` from both confirm paths, persists on history load/save.
• `components/files/UploadConfirmDialog.vue` — Public button re-enabled as a real radio option; emits `visibility-change` so the parent can re-quote (public batches cost one extra chunk).
• `pages/files.vue` — `quotedVisibility` tracks what the pre-quote represents; `onVisibilityChange` re-quotes when the user flips the selector; `confirmUpload` drops the pre-quote if its visibility no longer matches. Uploads table shows a blue "PUBLIC" badge — clicking copies the on-network address with a share-aware toast.

Test plan

• `vitest run` — 19/19 pass
• Nuxt typecheck clean on app code (only pre-existing Tauri build-artifact noise)
• Post-EVM Network Naming from Connect Wallet/MetaMask #39 merge: bump `ant-core` rev, `cargo check` resolves `Visibility` / `file_prepare_upload_with_visibility` / `FileUploadResult.data_map_address`
• Manual: upload a file with Public selected → verify a 64-char hex address appears on the row with a PUBLIC badge → click it → verify clipboard contents
• Manual: upload with Private selected → verify row still shows the datamap filename (existing behaviour unchanged)
• Manual: fresh download of a public upload via "Download by Address" using the copied public_address → verify `download_public` fetches the DataMap off-network and decrypts correctly

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