NumberForm foundation 3: BigNumberPad primitive and layout composition #98171

Sub-issue of #97970.
Migration plan: jmgraa comment.

Last foundation issue. It adds the number pad primitive, composes every primitive through NumberForm across all four layout combinations, and rewires NumberWithSymbolForm to delegate to that composition.

Scope

• Step: 3 of the migration plan
• Scope: 1 new primitive, layout composition, NumberWithSymbolForm becomes an adapter

Target API

<NumberForm value={amount} onInputChange={handleAmount}>
    <NumberForm.Input symbol="$" position="prefix" />
    <NumberForm.FlipButton />
    <NumberForm.CurrencyButton currency="USD" onPress={handleCurrency} />
    <NumberForm.BigNumberPad onNumberPressed={handleNumber} />
</NumberForm>

NumberForm.BigNumberPad wraps the existing src/components/BigNumberPad.tsx rather than reimplementing it.

Layouts to cover

Four combinations, currently spread across three early return paths in src/components/NumberWithSymbolForm.tsx:

• portrait, touch
• portrait, non touch
• landscape, touch
• landscape, non touch

Duplication being collapsed:

• BigNumberPad is rendered twice, at lines 686 (landscape) and 785 (portrait), with identical props. The landscape branch also has a redundant double shouldShowBigNumberPad ? check at lines 680 and 685.
• The ref fanout closure is duplicated verbatim at lines 551-559 and 587-595.
• Lines 581 to 635 build a shared textInputComponent with roughly 35 props, used by the landscape and portrait paths but not by displayAsTextInput.

Correction to step 3 of the migration plan

The plan says step 3 should "Remove the remaining duplicated layout/button paths from the old component". Do not do that literally. NumberWithSymbolForm stays in use until the cleanup issue #98178, and roughly 30 files depend on it transitively through AmountForm, MoneyRequestAmountInput and MoneyRequestAmountForm. Gutting its layout paths here breaks all of them.

Instead: rewrite NumberWithSymbolForm as a thin adapter that renders the NumberForm composition internally. Its public props and its ref keep working unchanged, the duplicated layout and button code disappears, and no call site has to move yet. Deletion happens in #98178, after every consumer migrates.

Out of scope

• Marking anything @deprecated, and the migration guide — #98172
• Any call site change
• Deleting NumberWithSymbolForm, its props types or its story — #98178

Acceptance criteria

• NumberForm.BigNumberPad wraps src/components/BigNumberPad.tsx
• NumberForm composes correctly in portrait and landscape, touch and non touch
• NumberWithSymbolForm renders the NumberForm composition internally and keeps its current public props and ref
• No duplicated pad, button, FormHelpMessage or ref fanout code remains in NumberWithSymbolForm
• Pad and layout tests ship with this PR
• The tests from #98168 still pass with no edits, which is the proof the adapter is behaviour preserving
• Spot check the three heaviest screens by hand: the IOU amount page, the manual distance page and the confirmation list amount field, in both orientations
• Verify that no errors appear in the JS console

Dependencies

• Blocked by: #98170
• Blocks: #98172