mecatui: click-to-open URLs in scrollback

Split out of #486 (mouse interactions). The first PR ships clickable approval/plan-review buttons; this is the follow-up for click-to-open links in the conversation scrollback.

Behaviour

A single left-click (press+release, no drag) on a URL in the conversation scrollback opens it in the platform browser. Drag-selection, double-click word-select, and triple-click line-select are unchanged — the open fires only on a clean single click (clickCount==1 + empty selection at release, the existing onMouseRelease empty branch in cmd/mecatui/ui/update.go).

Trust boundary (load-bearing)

Scrollback content is model-influenced and untrusted. The URL must be:

• parsed with net/url.Parse (never regex-guess the scheme),
• gated on a scheme allowlist {https, http, mailto} (the allowlist lives in ui, the consumer),
• passed to the opener as ONE direct argv element — NEVER through a shell (sh -c would be command injection on file:///sh: schemes),
• length-clamped (e.g. 2048) and control-byte-rejected.

Shape (from the #486 plan)

• client/urllaunch.go (new): URLOpener interface + NewURLOpener() probing xdg-open/open/rundll32 — mirrors client/clipboard.go's probe + injected-runner pattern. os/exec stays in client, never in ui.
• ui/urls.go (new): urlAtLine(stripped, col) tokenizer (grapheme-walk like wordAt), scheme allowlist, openURLCmd (async tea.Cmd like shellWriteCmd), muted status on success/failure.
• Deps gains Opener client.URLOpener (nil disables).
• OSC8 hyperlink RENDERING is explicitly out of scope (would break View goldens + interfere with styleSelection ANSI splicing); click-to-open works by hit-testing text against a URL scan at click time, independent of rendering.

Known v1 limitation: a URL soft-wrapped across two viewport lines is only detected from the clicked line's fragment.