feat(app): render terminals via WebGL, attached only to the active pane

[fujibee]

Summary

• Root cause of issue Desktop app (Windows): input cursor flickers with Codex CLI output in the web-based terminal pane #383's reported flicker: koit determined the original report was the Codex CLI's animated "Working (Ns...)" status text, not the input cursor. Investigation traced this to xterm.js's default DOM renderer, which does a full replaceChildren() DOM-node rebuild of an entire terminal row on every content update — its own source explicitly calls this renderer "not meant to be particularly fast" and describes it as "a fallback for when the webgl addon is slow." A CLI repeatedly recoloring the same status line forces that expensive rebuild on every frame.
• Adds @xterm/addon-webgl (0.19.0) as the primary renderer. This is a real fix, not a workaround — it's xterm.js's own intended fast path — and also a general performance improvement for all terminal output, not just this one animation.
• Scoped to the currently active pane only, not mount/unmount: every pane across every tab/team stays mounted for the app's whole session (live PTY + scrollback), so eagerly attaching WebGL to every mounted pane would exhaust the browser's concurrent-WebGL-context cap (Chromium: ~8-16) well before a typical multi-pane session's real pane count. TerminalPane gets a new required active prop; a dedicated effect attaches WebglAddon when the pane becomes active and disposes it (releasing the GPU context) when it goes inactive.
• Handles real GPU/driver context loss via onContextLoss: disposes and falls back to the default DOM renderer for the rest of that pane's active session — no regression on unsupported/locked-down GPUs, no retry loop.

Test plan

• tsc --noEmit clean
• pnpm test — 152 passed (no new tests: the added logic is a thin, non-pure attach/detach effect over real xterm/WebGL objects, consistent with this repo's convention of only unit-testing extracted pure logic — flagging for co1 in case a different call is warranted here)
• Manually verified live in tauri dev by koit: no rendering breakage or hangs across 3 teams / ~10 panes, switching tabs/teams repeatedly. Visual comparison of shimmer smoothness and the DOM->WebGL anti-aliasing change (ClearType-ish -> more grayscale) also confirmed fine by koit directly.