fix(web): sync idle decay from settings

[zerone0x]

Description

• fetch /api/settings once on app boot and derive the web idle-decay window from theme.idle_decay_minutes
• thread that resolved window through workspace grouping, dashboard stats, and status glyph/class helpers so fresh-idle matches the configured server value instead of a hard-coded mirror
• add unit coverage for the settings parsing helper and keep the existing freshness helpers covered with the explicit window path
• closes web: sync idle_decay_minutes from /api/settings instead of hard-coded mirror #874

Verification:

• cd web && npm run test:unit -- src/lib/session.test.ts
• cd web && npm run build

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:
Codex (GPT-5)

Any Additional AI Details you'd like to share:
Implemented the /api/settings wiring for the web idle-decay signal directly from issue triage for #874.

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