perf(graph): lazy-load optional editor UI and styles

[im-ian]

Summary

This keeps the optional Graph editor out of the initial application payload while preserving its existing creation and session workflows.

1. Lazy Graph entry points — Load the Graph creation dialog only when requested and the Graph session view only when a Graph session is active.
2. Shared async editor chunk — Let Vite extract the React Flow editor and preset toolbar used by both Graph entry points.
3. Deferred Graph styles — Move the React Flow stylesheet from the global application CSS into the lazy Graph editor dependency.
4. Runtime coverage — Exercise Graph creation, editing, execution, saved-session rendering, and ordinary Chat behavior after the split.

Expected impact

Asset	Before	After	Change
Initial JavaScript	2,981,678 B	2,731,645 B	-250,033 B (-8.4%)
Initial JavaScript, local gzip	842,124 B	766,776 B	-75,348 B (-8.9%)
Graph shared JavaScript	Included initially	237,220 B async chunk	Deferred until Graph use
React Flow CSS	Included initially	15,413 B async CSS	Deferred until Graph use

Design notes

• The Sidebar dialog and Pane session view are separate lazy boundaries because they are independent Graph entry points.
• Both boundaries resolve named exports without changing the public component modules.
• Suspense fallbacks are intentionally empty: Tauri serves packaged assets locally, and existing layout remains stable during the short first load.
• Graph dialogs now mount only while open; their existing open effect already resets draft and capability state for every invocation.

Test plan

• pnpm run typecheck — passed
• pnpm run test — 1,290 tests passed
• pnpm exec playwright test tests/e2e/graph-editor.spec.ts — 6 tests passed
• pnpm run test:e2e — 318 tests passed
• pnpm run build — passed and emitted separate Graph JS/CSS chunks
• git diff --check — passed

Notes

• The build still reports the pre-existing main-chunk size warning and ineffective dynamic-import warnings for api.ts and store.ts; this PR materially reduces the main chunk but does not address those separate boundaries.

[im-ian]

Merge readiness note: this PR is technically mergeable now.

• GitHub reports MERGEABLE / CLEAN.
• CI is green: Frontend, E2E (shard 1/2), and E2E (shard 2/2) passed.
• Self-review is complete with no blocking findings.
• perf label is applied.
• Production build evidence confirms the initial JavaScript payload drops by 250,033 bytes (8.4%), with Graph JavaScript and React Flow CSS emitted as async chunks.

Proceeding to merge because the user explicitly requested the reviewed release fixes to be merged.