fix(tipping): cap the tip preset buttons at three digits

[bmc08gt]

The preset buttons split the tip modal's width three ways and write the amount out in full. In USD the tiers are $5 / $10 / $20 and fit; localized to a low-value currency they do not. The same tiers are 7,500 / 15,000 / 30,000 pesos, and a custom amount can be larger still.

Fiat.abbreviated() caps the visible amount at three digits. Under 1,000 it formats exactly as before; above that it scales to K/M/B/T, keeping only the decimals the cap leaves room for and dropping trailing zeros. It rounds to significant digits before choosing the scale, so 999,999 prints as $1M rather than $1,000K.

	written out	on the button
ARS	7,500 / 25,500 / 123,456	7.5K / 25.5K / 123K
VND	126,000 / 1,315,000	126K / 1.32M
IDR	332,000	332K
JPY	750 / 3,000	750 / 3K
USD	$5 / $12.50	unchanged

The button's content description keeps the full amount, so the abbreviation is visual only and TalkBack still reads "332,000".

The helper formats through Fiat, so the currency symbol, grouping, and per-currency fraction digits are preserved. FiatTest covers the scale boundaries, the decimal cap, the carry into the next scale, and the currencies above.

Discovery uses the same rule

The second commit moves discovery's market caps, holder counts and deltas onto it. They previously went through Number.abbreviated(), which had its own rule — two decimals at every scale, so a delta could run to five digits (+$999.99K) — and two defects that the shared implementation does not have:

• The scale came from the unrounded value, so a delta of 999,999 printed as $1000K.
• The scale thresholds were >= comparisons on the signed value, so no negative ever matched one. A holder delta of -12,345 rendered in full, even though the call site's comment expects the sign to carry through an abbreviation.

Both helpers now share the rounding and scale selection; only the final formatting differs, because Fiat needs the currency symbol and Number does not. Discovery reads 1.23K / 693K / -12.3K.

One behaviour change below a thousand: values are now rounded instead of truncated, so a fractional market cap delta of 42.7 shows as 43. Holder counts are whole, so this only reaches market cap figures.

Unchanged by this PR, but worth knowing: ARS, IDR and COP resolve no single-character symbol through CurrencyCode.singleCharacterCurrencySymbol, so those buttons show bare digits. Full formatting drops the symbol too, but it reads as more of a gap now that the number is short.

[bmc08gt]

Deliberately not identical to the iOS side, code-ios-app#704, which does the same job for the Send a Tip chips. That one keeps a decimal only while the scaled figure is a single digit; this keeps three significant digits. The same preset therefore reads $1.23K / $25.5K here and $1.2K / $26K there.

Two smaller differences, same decision: this abbreviates the custom "…" slot as well, where iOS leaves it formatting exactly, and Number.abbreviated() (market cap in Discovery) stays its own rule, where iOS folded CompactCurrencyFormatStyle onto the new one.