feat(settings): add configurable default effort level

[reverb256]

Summary

Resolves #1846

Adds a "Default effort level" setting so users can configure the default reasoning effort for new tasks instead of always defaulting to "high".

Problem

There was no way to configure a default effort level in PostHog Code. Users who preferred a specific effort level had to manually change it every time they started a new task. The effort level always defaulted to "high" with no persistence beyond "last used".

Solution

Mirrors the existing defaultInitialTaskMode pattern — a setting that controls whether new tasks use a fixed value or "Last used":

• defaultReasoningEffort setting in settingsStore with type DefaultReasoningEffort = "low" | "medium" | "high" | "xhigh" | "max" | "last_used"
• Default value: "last_used" (preserves existing behavior for all users)
• Applied in usePreviewConfig when resolving the initial effort for new sessions
• Applied in the model-change handler fallback when the effort option needs re-resolving
• UI: New "Default effort level" select in Settings > General > Input section

Files changed

File	Change
settingsStore.ts	Add DefaultReasoningEffort type, defaultReasoningEffort state + setter, persisted
usePreviewConfig.ts	Use defaultReasoningEffort to resolve initial effort (mirrors defaultInitialTaskMode logic)
GeneralSettings.tsx	Add "Default effort level" setting row with analytics tracking

How to test

1. Open Settings > General > Input
2. Change "Default effort level" from "Last used" to e.g. "Max"
3. Start a new task — effort selector should default to "Max"
4. Change it to "Last used" — effort selector should remember the last value you used
5. Switch models — effort fallback should respect the configured default

Screenshots

Setting appears right below "Initial task mode" in the Input section:

Input
├── Initial task mode      [Plan ▾]
├── Default effort level   [Last used ▾]   ← NEW
├── Send messages with     [Enter ▾]
└── ...

[greptile-apps]

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