fix: avoid Array.prototype.at() to support the declared browser range

[mrholek]

Completes #565.

Array.prototype.at() requires Chrome 92 / Safari 15.4 / iOS 15.4, but the committed .browserslistrc declares Chrome >= 60, Safari >= 12, iOS >= 12, and the build only transpiles syntax — it does not polyfill (.babelrc.js has no useBuiltIns). So on the supported older browsers .at() is undefined and throws.

#565 fixed only the two calendar usages. This replaces all .at() calls in js/src:

• util/calendar.js — weeks.at(-1) → weeks[weeks.length - 1] (×2)
• chip-input.js — chips.at(-1) / parts.at(-1) → index access (×4)
• chip-set.js — chips.at(targetIndex) → chips[targetIndex < 0 ? chips.length + targetIndex : targetIndex] (preserves negative-index semantics; _navigateToEdge is called with 0 and -1)

Behavior is unchanged; existing chip/calendar suites pass (2994). Original calendar fix by @vladrusu (#565).

[coveralls]

Coverage Report for CI Build 29295672217

Coverage increased (+0.02%) to 94.655%

Details

• Coverage increased (+0.02%) from the base build.
• Patch coverage: 7 of 7 lines across 3 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	7462
Covered Lines:	7187
Line Coverage:	96.31%
Relevant Branches:	3090
Covered Branches:	2801
Branch Coverage:	90.65%
Branches in Coverage %:	Yes
Coverage Strength:	805.57 hits per line

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