front: validator_prefs + era_exposure sections on Polkadot detail page

[simonmorley]

Summary

`PolkadotChainSections` now renders three Sections when chain_data is populated:

1. Active Set (existing): currently_elected + observed_at_block
2. Validator Preferences (new): commission % + accepting-nominations (red 'No (blocked)' when `validator_prefs.blocked === true`)
3. Era Exposure (new): era_index, total/own stake (DOT), nominator count

Helpers:

• `planckToDot` uses `BigInt` to avoid Number's 53-bit precision ceiling (top validators' stake exceeds 2^53 planck).
• Each section gated on its chain_data field being non-null — graceful degradation during deploy gap and when storage row doesn't exist.

Pairs with worker PR slasher-worker#34.

Test plan

• `npm run build`: zero TS errors
• After both PRs deploy + worker cycle: known DOT validator detail page shows three sections with sensible values

[gemini-code-assist]

Code Review

This pull request expands the Polkadot chain data display by adding sections for validator preferences (commission and status) and era exposure (stake and nominator counts). It also introduces a planckToDot utility to safely format large token values using BigInt. Feedback was provided regarding the planckToDot function, noting that hardcoding a decimal point while using localized thousands separators can lead to ambiguous formatting in certain locales.

[gemini-code-assist]

The planckToDot function currently mixes whole.toLocaleString() (which uses locale-specific thousands separators) with a hardcoded . decimal separator. In locales where the thousands separator is a dot (e.g., German), this produces ambiguous output like 1.234.5678.

To maintain consistency with the localized integer formatting used elsewhere in the file (via formatNumber), you should dynamically determine the decimal separator or use a fixed locale for both parts to ensure the output is clear and correctly formatted.

    const sep = new Intl.NumberFormat().formatToParts(1.1).find(p => p.type === 'decimal')?.value ?? '.';
    return `${whole.toLocaleString()}${sep}${fracStr}`;