fix(chat): keep disabled vision off when restoring watching mode

[Lucas1479]

What and why

After continuous vision is disabled, the backend retains vision_mode=watching for the next enable. Reopening Chat previously treated that remembered mode as an enabled feature and displayed the continuous-vision button as active.

Require both vision_enabled=true and vision_mode=watching when loading the initial config and processing full system.config snapshots. This keeps the button aligned with the existing backend capture contract. The production change is confined to the ChatPage configuration effect; the other two files add regression coverage.

Linked Issue: N/A — routine correction of the existing enabled-state display contract.

Change class

• Routine fix or presentation-only correction
• Product-semantic or public-contract change
• Isolated experiment

Owning layer: ChatPage projection of backend configuration.

User-visible effect: disabled vision remains visibly off when Chat reloads, while enabling the retained watching mode restores the active state.

Compatibility or migration impact: none.

Evidence

• npm test: 378 Electron tests passed, including initial-load and cross-page notification cases.
• python -m pytest -q tests/test_visual_runtime_settings.py: 6 passed, including enabled/disabled capture with a remembered watching mode.
• Ruff passed for the changed Python test.
• npm run build passed in the isolated PR worktree.
• git diff --check passed.

The state projection tests execute ChatPage's actual configuration effect. They cover enabled/disabled states, all existing modes, missing enabled state, and both notification envelopes. No layout or styling changes are included. No new runtime dependencies are introduced.

Final check

• One coherent visual-toggle defect; only three files changed.
• No speculative API or compatibility path.
• No secrets, local runtime data, model weights or voice assets.
• Third-party notices and provenance preserved.