fix(ui): show correct duration for fee rate

[pwltr]

Description

Mostly a refactor/cleanup but also adds a way to override fee estimates in debug mode (useful for UI work) and aims to show estimated confirmation time when a custom fee is set, instead of "Depends on fee".

• show correct duration for fee rate where applicable (transaction speed settings, send fee rate, boost sheet)
• Route fee estimation usage through FeeEstimatesManager; remove direct blocktank.fees() calls
• Activity: design updates v59 (activity icons, text)
• force .modern, .bitcoin denomination for custom fee UI
• add "override fees" dev setting to be able to debug fee estimation UI
• update localization keys and strings
• ActivityListViewModel/TransferViewModel: drop unnecesary fee estimation refresh (payOrder takes required satsPerVbyte, estimates cache warmed in ActivityList/ActivityLatest .task)

Screenshot / Video

Simulator.Screen.Recording.-.iPhone.17.-.2026-03-05.at.17.29.39.mov

[pwltr]

Drafted to give precedence to other PRs

[claude]

Bug: raw value change breaks existing UserDefaults data and migration path

Changing from explicit raw values to implicit ones changes the stored string representation:

// Before: raw values were "Bitcoin" and "Fiat"
case bitcoin = "Bitcoin"
case fiat = "Fiat"

// After: raw values are now "bitcoin" and "fiat" (lowercase)
case bitcoin
case fiat

MigrationsService.swift writes the old capitalized values to UserDefaults:

bitkit-ios/Bitkit/Services/MigrationsService.swift

Lines 1210 to 1214 in d9f3544

	}
	if let unit = settings.unit {
	let nativeValue = unit == "BTC" ? "Bitcoin" : "Fiat"
	defaults.set(nativeValue, forKey: "primaryDisplay")
	}

After this change, @AppStorage("primaryDisplay") var primaryDisplay: PrimaryDisplay will fail to decode "Fiat" (stored by the migration) because PrimaryDisplay(rawValue: "Fiat") now returns nil. The AppStorage property wrapper will silently fall back to the default value .bitcoin, resetting any user who had fiat as their primary display preference.

The migration write path is also permanently broken going forward — it still writes "Bitcoin" / "Fiat", which will never decode correctly with the new enum.

Fix: Either restore the explicit raw values, or update MigrationsService.applyRNSettings to write the new lowercase values ("bitcoin" / "fiat") and add a migration step to convert existing stored values.