refactor(webview): extract shared safePostMessage helper

[mtskf]

Summary

Consolidates ~9 near-identical webview→host postMessage try/catch blocks into one shared safePostMessage helper.

Changes

• Add src/webview/safe-post-message.ts: safePostMessage(host, message, label, onError?) wraps the post + console.error("[quoll] postMessage(<label>) failed", err) and returns a success boolean; an optional onError hook lets a caller (e.g. postEditMessage) read the raw error for its own cleanup.
• Migrate all 9 call sites to the helper: editor.ts (postEditMessage / postLintDiagnostics / postCaretReport), cm/context-handoff.ts (×2), cm/switch-editor.ts, cm/open-external.ts, cm/link-handlers.ts, cm/image/image-paste.ts.
• Each call site keeps its own post-catch cleanup (dispatch serialize-error, clearPending, return value) — mechanical extraction only, no behaviour change. Log text is byte-identical.

Related

• TODO: Extract a shared safePostMessage helper (webview) — simplification sweep 2026-07-10

Test Plan

• pnpm compile — clean
• pnpm test:unit — 3021/3021 passed
• pnpm build + pnpm package + code --install-extension --force — installed and verified