Add configurable local transaction fee rate

[rohenaz]

Desktop currently applies a fixed 250 sats/kB rate to every locally funded transaction. That default was introduced as a margin for reported underfunded token ancestors; the fee calculation does not select it based on ancestor confirmation status. A 4,901,246-byte inscription funded by an already-confirmed input therefore paid 1,225,312 sats despite an advertised Arcade floor of 100 sats/kB.

This draft adds one optional custom rate per network under Settings → Transaction fees:

• Show the configured rate and the selected network’s Arcade policy floor.
• Require a fresh, bounded policy lookup when saving a custom rate, and reject below-floor values in both the UI and main process.
• Persist the override locally and apply it after restart to transaction storage and its separate monitor instance. With no override, existing defaults remain unchanged.
• Explain the limitation for unconfirmed ancestors and remote-storage wallets. Include translations for all supported languages.

This is control over the existing fixed rate, not automatic ancestor-aware pricing. No ancestor classification or fee-expansion mechanism is removed. Lowering the fixed rate can still leave a package with underfunded ancestors unable to confirm; calculating that shortfall is a separate design task. The draft does not change the default rate, broadcaster selection, or wallet-toolbox dependency.

Validation: 37 configured tests pass (including 13 fee tests), renderer and Electron typechecks pass, and the Electron build succeeds. Isolated UI checks with a mocked IPC bridge cover below-floor blocking, successful saving, a policy increase during save, and policy outages. A read-only live check resolves the advertised floor to 100 sats/kB. No wallet settings were changed and no transactions were broadcast during validation.

Related prerequisite investigation: Arcade draft #338 adds passing regressions showing that an under-floor parent is rejected even when its child covers their combined fee. The reviewed Teranode v0.16.0-beta-9 and main sources enforce the fee floor before block assembly and do not implement CPFP fee aggregation; legacy Bitcoin SV has a separate secondary-mempool CPFP path. A higher fixed child rate therefore does not universally rescue underfunded ancestors. This Desktop draft remains a manual-rate control, not a solution to that acceptance gap. No production change in this draft has been expanded on the basis of the investigation.