refactor: AmountInputViewModel to @Observable

[CypherPoet]

Depends on

This PR is based on #2's branch (fix/amount-screens-cap), and transitively on #4. It targets that branch as its base, so the diff below shows only this PR's changes. Land #4 then #2 first, then re-target this to master.

Description

This PR migrates AmountInputViewModel from the legacy ObservableObject pattern to the @observable macro, matching the project's stated SwiftUI direction. There is no behavior change.

The view model swaps ObservableObject and @published for @observable, NumberPadTextField drops @ObservedObject, and the eight amount-entry screens move from @StateObject to @State. No @bindable is needed anywhere, since nothing creates two-way bindings to the view model; every consumer reads its values or calls its methods.

Linked Issues/Tasks

Follows up synonymdev#346 and the amount-screen cap work.

Screenshot / Video

N/A; no behavior change.

QA Notes

Manual Tests

• 1. regression: Send -> Amount: typing updates the displayed amount and fiat conversion; the cap still works.
• 2. regression: Receive -> Amount: typing updates the invoice amount.
• 3. regression: Transfer -> Spending Amount: typing updates and caps as before.
• 4. regression: LNURL Pay / Withdraw -> Amount: typing and cap behave as before.

Automated Checks

• Built app-wide for iOS Simulator (Debug) locally; the whole app target compiles against the migrated view model and BitkitTests/NumberPadTests passes (24 tests).
• No coverage changed: pure refactor (about 13 mechanical line edits across 10 files).
• CI: standard build/test checks run by the PR bot.

[CypherPoet]

Re-opened upstream as synonymdev#586. Closing this fork self-review PR.