dx: add .editorconfig (292/294/297 already resolved on main)

[ndii-dev]

Closes #292
Closes #294
Closes #297
Closes #301

#292 — client vesting math diverges from contract formula

Already resolved on main — lib/stream-utils.ts's getUnlockedAmount now computes (elapsed * stream.linearAmount) / stream.duration, matching the contract's multiply-before-divide order exactly, with a doc comment calling out not to use amountPerSecond.

#294 — no coverage thresholds in vitest config

Already resolved on main (commit bd05c85) — vitest.config.ts's coverage block now sets thresholds (statements/branches/functions/lines) with a comment noting they're a ratcheting baseline.

#297 — no WebKit project in Playwright config

Already resolved on main — playwright.config.ts now includes a webkit-desktop project alongside chromium/firefox.

#301 — no .editorconfig at repo root

Added .editorconfig matching the project's Prettier conventions (2-space indent, LF line endings, UTF-8, trim trailing whitespace, final newline; markdown exempted from trailing-whitespace trimming).

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