cardano-wasm demo: fee estimation, balance checks, signing, tx export

[palas]

This PR adds the following functionality to the cardano-wasm demo:

• Conway/Dijkstra era toggle; estimateMinFee over the described tx
• balance arithmetic in Elm: change = inputs - outputs - fee; explicit outputs and the change must meet the ADA-only min-UTxO, gating the sign button (insufficient/dust/too-small states)
• the two numbers Elm needs come as init flags from web/pparams.js — the same pinned ledger-format object estimateMinFee consumes (single source)
• signing returns the CBOR and the tx id (body hash, shown pre-submission)
• export: cardano-cli TextEnvelope download (tx.signed) + copy CBOR, with an era-matched broadcast hint and the witness counts it was signed with
• stale protection: shape edits reset the fee (invalidateShape), and the fee/sign replies are accepted only while their request is still live
• live tx inspector mirrors the exact spec, including the implicit change

Context

This is a follow up of: #1278

How to trust this PR

• Diff is entirely in cardano-wasm/demo/ + a changelog fragment; CI already compiles it and checks elm-format.
• This is the first slice that produces a real signed transaction — but nothing is broadcast: signing happens in cardano-wasm in the browser, keys stay in memory, and the only outputs are a downloaded tx.signed and copied CBOR.
• You can test it by serving the contents of the demo artifact generated by the CI.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Pull request overview

This PR extends the cardano-wasm/demo transaction builder from “draft spec” into a workflow that can estimate fees, perform basic balance/min-UTxO checks, sign a transaction in-browser, and export the signed transaction (CBOR + cardano-cli TextEnvelope), with additional UX support such as era toggling and a live tx inspector.

Changes:

• Add pinned protocol parameters (pparams.js) and pass fee-related values into Elm via init flags (single source of truth for fee estimation + Elm balance arithmetic).
• Implement fee estimation + signing ports (JS) and corresponding Elm commands/decoders/state gates (fee state, tx state, staleness protection).
• Add UI for era selection, summary/balance display, signing/export actions, and a “live tx inspector”.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
cardano-wasm/demo/web/pparams.js	Introduces pinned ledger-format protocol parameters consumed by fee estimation and Elm flags.
cardano-wasm/demo/web/ports.js	Adds unsigned-tx builder plus fee estimation/signing port handlers; passes protocol flags into Elm.
cardano-wasm/demo/src/Wasm.elm	Adds ports-side commands for fee estimation/signing and encodes the tx “spec” JSON sent to JS.
cardano-wasm/demo/src/View.elm	Adds era toggle, builder summary, signing/export UI, and a live tx inspector panel.
cardano-wasm/demo/src/Update.elm	Adds fee/sign/export state machine, staleness handling, and cardano-cli TextEnvelope download.
cardano-wasm/demo/src/Types.elm	Adds era/fee/tx state types, signed payload types, and Protocol flags record.
cardano-wasm/demo/src/State.elm	Adds balance/min-UTxO logic, readiness gates, staleness helpers, witness counting, and Protocol-flag init.
cardano-wasm/demo/src/Ports.elm	Declares new fee-estimation and signing ports.
cardano-wasm/demo/src/Net.elm	Adds era tags and cardano-cli TextEnvelope type mapping per era.
cardano-wasm/demo/src/Main.elm	Switches Elm program flags from () to Protocol and wires new incoming ports.
.changes/20260718_cardano_wasm_demo_signing.yml	Changelog fragment documenting the demo feature addition.

Files not reviewed (1)

• cardano-wasm/demo/web/pparams.js: Generated file

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