feat: Export depth options (Export first thought / Export subthoughts)

[Copilot]

Adds two advanced checkboxes to the Export Modal's "Advanced" section to control export depth, both defaulting to checked (preserving existing behavior).

New options

• Export first thought — when unchecked, skips the selected thought(s) and exports only their descendants. For multi-selection, skips the entire first level.
• Export subthoughts — when unchecked, exports only the top-level thought(s) with no descendants. Traversal stops early rather than post-processing.

Export first thought	Export subthoughts	Output for A → [B → [D], C → [E]]
✅	✅	A, B, D, C, E (default)
❌	✅	B, D, C, E
✅	❌	A
❌	❌	B, C

Changes

• src/selectors/exportContext.ts — adds maxDepth?: number option; maxDepth=0 stops recursion immediately, propagates as maxDepth - 1 through recursive calls
• src/components/modals/Export.tsx — adds shouldExportFirstThought / shouldExportSubthoughts state; when first thought is excluded, expands sortedPaths to children IDs before mapping to exportContext; numDescendantsInState short-circuits to 0 when subthoughts are excluded
• src/selectors/__tests__/exportContext.ts — adds tests for maxDepth: 0 and maxDepth: 1 for both plain text and HTML formats

[github-actions]

Vercel preview: https://em-5eue0kgyy-cybersemics.vercel.app

[raineorshine]

@copilot You forgot to push ad5cee4 to this branch.

[Copilot]

@copilot You forgot to push ad5cee4 to this branch.

Applied in commit feat: add Export first thought and Export subthoughts advanced options. All 15 exportContext tests pass and lint is clean.