fix(server): stop misdiagnosing missing shared libs as missing Chromium - #746

What & why

Refs #691. That issue bundles four distinct misdiagnoses in Reticle's pooled-lease
Chromium launch path. This PR fixes one of them — the rest are out of scope here
and remain open on the issue:

• ✅ fixed by this PR: a launch failure caused by a missing OS shared library (e.g.
  libnspr4.so) is misreported as "Chromium is not installed."
• ⬜ not fixed by this PR: Windows "package not installed" wording, a post-install
  build/version mismatch, and headed-vs-headless-only failures on Windows. Those have
  unverified root causes and are separate follow-up work.

playwrightLauncher() in packages/server/src/pool/playwright-launcher.ts catches every
chromium.launch failure with one regex
(/executable doesn.?t exist|playwright install|browsertype\.launch/i). Playwright
prefixes almost every launch error with browserType.launch:, so that regex also matches
Playwright's distinct "Host system is missing dependencies to run browsers" failure — the
browser binary is present, but the host's shared libraries are not. Users hitting that case
were told to run npx playwright install chromium, which succeeds and fixes nothing,
because the binary was never missing.

This adds a narrower check for that message, checked before the generic one, that
points at npx playwright install-deps chromium instead — pinned to the daemon's bundled
playwright version the same way the existing hint already is. A new
chromiumInstallDepsCommand sits next to the existing chromiumInstallCommand in
chromium-hint.ts, sharing a small internal pin helper.

Authored with AI pair-programming assistance (Claude Code) and reviewed by a human
before submission.

How it was verified

• Added a test that reproduces the bug on unmodified main: a mocked chromium.launch
  rejection carrying Playwright's real "Host system is missing dependencies to run
  browsers" message was, before this fix, rewritten into the missing-binary hint (RED).
  After the fix it correctly names install-deps and no longer mentions the
  missing-binary hint (GREEN).
• Added a regression test confirming the pre-existing missing-binary hint is unchanged,
  and a test confirming an unrelated launch failure is still rethrown as-is.
• Added unit tests for the new chromiumInstallDepsCommand, mirroring the existing
  chromiumInstallCommand coverage (pinned and unpinned cases).
• pnpm --filter @reticlehq/server exec vitest run on the two changed test files:
  16/16 passing.
• pnpm --filter @reticlehq/server run typecheck: clean.
• eslint run directly against the four changed source/test files (the full monorepo
  pnpm lint — which also builds/lints every fixture app — timed out in the sandbox this
  PR was prepared in; the scoped run against the changed files is clean, and CI runs the
  full gate regardless).

Gates run

• pnpm lint && pnpm typecheck && pnpm test:unit (~2 min — always) — run in
  package/file-scoped form locally (see verification notes above); CI runs the
  full monorepo command.
• pnpm test:e2e (~8 min) — not touched (no tool surface, packages/core, observer,
  or telemetry change)
• pnpm gate:install (~15 min) — not touched (no init/vite-plugin/next/
  babel-plugin change)
• pnpm test:e2e:desktop (~3 min) — not touched (no electron/tauri/desktop-capture
  change)
• None of the e2e/install/desktop tiers apply to this change

Checklist

• Every commit is signed off (git commit -s)
• Tests added/updated (RED → GREEN); the change is covered by a test that would fail
  without it
• No any, no free strings, no non-null !
• No console.log or internal tracking codes left in the diff
• Each changed file is under the 1000-line cap
• CHANGELOG.md updated under [Unreleased] → ### Fixed (user-facing behavior
  change)
• Security-affecting — n/a, no auth/redaction/trust-boundary change