fix(web): sync idle decay window from settings

[zerone0x]

Description

• fetch /api/settings on app boot and resolve theme.idle_decay_minutes into a shared web idle-decay window
• thread that window through dashboard/session activity helpers so fresh-idle indicators match the user's configured decay window
• add a small unit test for the settings-to-window conversion

Closes #874.

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure / CI

Checklist

• I understand the code I am submitting
• New and existing tests pass
• Documentation was updated where necessary
• For UI changes: included screenshot or recording

AI Usage

• No AI was used
• AI was used for drafting/refactoring
• This is fully AI-generated

AI Model/Tool used:

OpenClaw issue-hunter (GPT-5.4)

Any Additional AI Details you'd like to share:

Verification run in the clean temp clone:

• npm run test:unit ✅
• npm run build ✅
• npm run lint ⚠️ fails on pre-existing react-hooks/set-state-in-effect / unused-var errors in unrelated existing files (src/App.tsx, src/components/StatusGlyph.tsx, src/hooks/useFileDiff.ts, several Playwright specs)

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