fix(storage): keep cached settings unchanged when saving fails

[jonathanKingston]

A failed settings write currently updates the in-memory cache before persistence succeeds. After a disk-full or permission error, reads return a value that will disappear on restart.

Publish the cached snapshot only after the backing write succeeds. Regression coverage checks an existing key, a new key, and a successful retry; cloning semantics remain unchanged.

Validation: pnpm run check. The regression fails on current main and passes with this fix. Pure persistence logic; no DOM or layout changes.

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}