fix(gui): open external links in the default browser via tauri-plugin-opener

[zhouzhuozhzh-maker]

Problem

In the Tauri desktop shell, clicking a link in an assistant message (rendered as markdown) does not open the user's default browser. Instead, the anchor tag's target="_blank" attempts to navigate the webview itself, which either does nothing or opens inside the app window — not the system browser.

The root cause was two-fold:

1. No opener plugin: tauri-plugin-opener was not installed, so window.__TAURI__.opener was undefined and openExternal() in tauri.ts could only fall back to window.open() (which also navigates the webview, not the system browser).
2. No click interception: Markdown.tsx rendered links as plain <a target="_blank"> without calling openExternal(), so even if the opener plugin had been present, the links wouldn't have used it.

Fix

1. Wire up tauri-plugin-opener (Rust shell)

• Add tauri-plugin-opener = "2" to Cargo.toml
• Register tauri_plugin_opener::init() in lib.rs
• Grant opener:default permission in capabilities/default.json

2. Intercept link clicks in the frontend

• In Markdown.tsx, add an onClick handler that calls e.preventDefault() + openExternal(href) for external links, so the URL is handed to the opener plugin (which launches the default browser) instead of navigating the webview
• Falls back to window.open() in browser dev mode (where the opener plugin is not present)
• Updated the comment on openExternal() that said "the desktop webview has no opener plugin wired yet" — it is now wired

3. Tests

• Added a test verifying that clicking an ordinary markdown link calls openExternal with the correct URL
• All 109 existing frontend tests continue to pass

Files changed

File	Change
surfaces/gui/src-tauri/Cargo.toml	Add tauri-plugin-opener dependency
surfaces/gui/src-tauri/src/lib.rs	Register opener plugin
surfaces/gui/src-tauri/capabilities/default.json	Grant opener:default permission
surfaces/gui/src/tauri.ts	Update openExternal comment
surfaces/gui/src/components/Markdown.tsx	Intercept clicks → openExternal()
surfaces/gui/src/components/Markdown.test.tsx	Add click→openExternal test

Verification

✓ src/components/Markdown.test.tsx (4 tests) — 27ms
✓ 18 test files, 109 tests passed (2.53s)
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