fix(browser): honor POSIX PLANNOTATOR_BROWSER under WSL and warn on launch failure

[FNDEVVE]

Problem

With PLANNOTATOR_BROWSER explicitly set under WSL, the value was routed through cmd.exe /c start regardless of what it named, so a Linux path or script could never launch; the failure was swallowed by a bare catch and the ready output only printed the URL. A headless (agent-launched, no TTY) session then had every observable signal reporting success while the reviewer was never shown anything. The Pi runtime had the same branch and additionally reported opened: true on spawn errors. #1472

Change

• Both runtimes classify the configured value first: Windows targets (*.exe, C:\…, /mnt/<drive>/…) keep the cmd.exe route; POSIX paths (/…, ./…, ../…) and bare names that resolve to an executable on the Linux PATH are executed directly — also under WSL.
• The Bun runtime spawns the resolved absolute path: Bun 1.3's $ resolves bare command names against the PATH captured at process startup, so a bare name would otherwise be invisible to the shell. The end-to-end test pins this.
• When an explicitly configured browser fails, one stderr line names the executable and the error before the existing fallback (VS Code IPC / URL print) runs. The Pi runtime returns opened: false on spawn errors instead of claiming success.
• Non-WSL Linux and macOS paths are unchanged; the macOS branch recently fixed by fix(pi-extension): run script/executable PLANNOTATOR_BROWSER paths directly on macOS #1429 is untouched.

Verification

Bun 1.3.14, disposable HOME: packages/server/browser.test.ts + apps/pi-extension/server/network.test.ts → 49 pass / 0 fail:

• direct spawn of a POSIX path and of a bare PATH name under mocked WSL, asserting the target script received the URL as its argument;
• chrome.exe still routed through a cmd.exe shim (end-to-end in the Pi runtime, whose node:child_process spawn honors a mutated PATH on every Bun version; the Bun-side resolver tests pin the same routing decision, since a cmd.exe shim cannot be spawned through Bun 1.3's $ on a non-Windows host);
• a failing executable warns on stderr naming itself, and a missing one yields opened: false with the fallback intact.

On Bun 1.4.2 the only failures in these two files are the three pre-existing port-selection listener-count tests (baseline-verified before this change).

Fixes #1472

[backnotprop]

Reviewed in depth before merge: mapped the full browser-launch matrix on both runtimes and confirmed exactly one cell changes execution (WSL + POSIX PLANNOTATOR_BROWSER - the #1472 silent no-op, now actually invoked with the resolved absolute path), the new failure warning can never break a session (warn then IPC fallback then the URL lifeline; no exit paths), WSL detection is untouched so plain-Linux false-positive risk is unchanged, and revert-verification at three granularities confirms the tests genuinely guard both halves. The Pi-side truthful opened flag is a nice bonus - headless Pi users now get the open-this-URL notification instead of silent false success. Two cosmetic notes for a rainy day: the /mnt/ prefix classifier is broader than its comment (a /^/mnt/A-Za-z/ tighten would match it), and a non-daemonizing browser script blocks openBrowser until exit (pre-existing pattern). Merging.