Redraw the app as editorial content on glass controls

[michaelst]

Material was showing through everywhere: cards, list tiles, app bars, a navigation bar, dropdowns, ripples. This replaces it with the design system in mobile/lib/design, per the 2a token sheet - flat hairline-ruled content, every control a pane of glass, light and dark following the system.

The signature is that a budget's progress is its own bottom rule thickened to 2px, so a card is never needed to hold one. See mobile/docs/design.md.

What changes for the user

• Large titles that collapse into a blurred band; a tab bar that floats as a glass capsule with the list scrolling under it
• Selection is entered by long press, and a transaction swipes right to review, left to spend from
• Dropdowns became sheets, the date picker became the wheel, the budget type a segmented control
• Cupertino page transitions, bounce physics, pull to refresh, and haptics on every toggle and tab
• Credit cards moves under its own "From connected accounts" rule. listedBudgets is untouched, so web parity holds

Notes for review

• phosphor_flutter cannot build on Flutter 3.47, which made IconData a final class. The 16 glyphs are vendored under mobile/assets/icons from the same Phosphor set the web app ships, drawn with flutter_svg
• Adds flutter_slidable for the swipe actions
• test/design/layout_test.dart lays the whole app out at 390x844 in both appearances. The other tests run at 800x600, which hid all three bugs it caught: the month title overflowing, the budget name truncating against a Spacer, and the tab bar falling back to the framework's error text style for want of a Material above it
• Not verified on a device - the simulator runtime is not installed here. The screens were checked by rendering the widget tree at phone size with the system font loaded

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