feat: add transfer from savings button on empty spending screen

[jvsena42]

Related to #873

FIGMA

This PR adds a "Transfer From Savings" button to the Spending wallet empty state, shown when the user has on-chain savings balance. This helps users discover the transfer flow without needing to navigate to the Savings screen first.

Stacked PR on:

• feat: connection issues view #878

Description

• Adds a SecondaryButton with transfer icon to the Spending screen empty state
• Button is visible only when the spending balance is zero, there's no activity, and savings balance is greater than zero
• Navigates to the existing transfer-to-spending flow (intro or amount screen, based on whether the user has seen the intro)
• Reuses the existing localized string lightning__funding__button1 ("Transfer from Savings")

Preview

Screen_recording_20260402_141812.webm

QA Notes

1. Ensure you have on-chain savings balance but zero Lightning spending balance and no Lightning activity
2. Open the Spending wallet screen
3. Verify the "Transfer From Savings" button appears below the balance header
4. Tap the button and verify it navigates to the transfer-to-spending intro (first time) or amount screen (returning user)
5. With zero savings balance and zero spending balance, verify the button does NOT appear
6. With non-zero spending balance, verify the existing "Transfer To Savings" button still works as before

[ovitrif]

@jvsena42 Your intuition about the Box wrapper was right — see your original comment on #890:

I didn't use a Box because in the past caused some bugs either from fullWidth or from .weight(1f) in the PrimaryButton when used for the gradient effect.

The issue showed up in the BackupIntroScreen preview and on the Wallet Backup sheet — the SecondaryButton with Modifier.weight(1f) wasn't filling its weighted space because the inner OutlinedButton had no size constraint when fullWidth = false.

Fixed in 2780cc5 by adding propagateMinConstraints = true to the Box wrapper, which forces the OutlinedButton to match the Box's constraints in all cases.

Preview

Before	After

[ovitrif]

Summary of changes on this branch

Feature:

• Add "Transfer from Savings" button on empty spending screen (1b796a1e8)

Fixes (post-merge with master):

• Add frosted glass blur (Haze) to the new transfer-from-savings button on the savings screen (fd8f672de)
• Fix SecondaryButton weight(1f) layout regression caused by Box wrapper — added propagateMinConstraints = true (2780cc539)
• Remove unused import (2d9d0d358)

Housekeeping:

• Merge with master to pull in button polish (fix: polish buttons to match figma design #887) and Haze blur (fix: use haze for secondary button bg blur #890)
• Merge with feat/connection-issues-view to resolve base branch divergence
• Lint fixes and changelog backfill

[ovitrif]

tAck

Should be good to merge now, afetr the set of micro updates I did, most relevant being:
added haze blur to this button like on the other one of this screen: