🤖 feat: add auto-hide sidebar setting

[Neppkun]

Summary

Adds an Automatically hide sidebar toggle in Settings → General → Appearance, directly below Theme. When enabled, the left sidebar renders collapsed by default and only expands while the cursor is over it, reusing the existing transition-[width] duration-200 animation. When disabled, behavior reverts to the current user-toggled sidebarCollapsed state. The preference is persisted to ~/.mux/config.json end-to-end — backend reader/writer, oRPC getConfig + dedicated updateAutoHideSidebar route, onConfigChanged fan-out, and a localStorage mirror for flash-free synchronous reads at boot — mirroring the chatTranscriptFullWidth flow exactly.

Background

The left sidebar is a useful workspace navigator but takes up screen real-estate that some users (especially on smaller displays or when focused on a single workspace) would rather reclaim while keeping the sidebar one mouse-flick away. The existing manual collapse hotkey/toggle is per-window state; this adds a persistent "auto" mode so users don't have to remember to expand/collapse it on every context switch.

Patterned after #3350 (full-width chat transcript) so the persistence/sync model is identical and easy to review.

Implementation

• Config schema + types (appConfigOnDisk.ts, common/types/project.ts): new optional autoHideSidebar?: boolean.
• Backend (node/config.ts): reader parses the new key via parseOptionalBoolean; writer follows the "only persist when true" convention so disabling the toggle removes the key from disk entirely.
• oRPC (common/orpc/schemas/api.ts, node/orpc/router.ts): getConfig echoes autoHideSidebar; new updateAutoHideSidebar route uses the shared booleanToggleRoute schema. onConfigChanged fan-out covers cross-window sync generically.
• Hook (browser/hooks/useAutoHideSidebar.ts): verbatim mirror of useChatTranscriptFullWidth — seeds from localStorage to avoid a flash on boot, refreshes via getConfig, subscribes to onConfigChanged, and uses a fetchVersion nonce so Settings writes can't be clobbered by an in-flight getConfig.
• Effective collapse boundary (browser/App.tsx): the persisted sidebarCollapsed is intentionally not overwritten while auto-hide is on. Instead, App.tsx computes effectiveSidebarCollapsed = autoHideSidebar ? !sidebarHovered : sidebarCollapsed and passes that to <LeftSidebar>. This way the user's manual collapse preference is preserved for when auto-hide is later turned off.
• Hover detection (browser/components/LeftSidebar/LeftSidebar.tsx): new optional autoHideEnabled + onHoverChange props attach onMouseEnter/onMouseLeave to the outer sidebar div only when auto-hide is on. The existing transition-[width] duration-200 animates the resulting width change for free — no JS animation needed.
• Settings UI (features/Settings/Sections/GeneralSection.tsx): new Switch row directly below Theme, with its own load-nonce + serialized update chain, exactly matching the chat-transcript pattern already in this file.

Edge cases

• Resize handle: lives inside the sidebar, so dragging keeps the cursor inside and the sidebar stays expanded for the duration of the drag.
• Touch/mobile: onMouseEnter/Leave don't fire on touch, so the toggle is effectively a no-op on mobile — mobile already auto-collapses via the existing isMobile seed and overlay backdrop.
• First paint: localStorage seed avoids any flash before the backend getConfig resolves.
• Keyboard/focus accessibility: hover-only expand is a known limitation; left as a follow-up if it grows in scope.

Validation

• make typecheck ✅
• make lint ✅
• make static-check ✅ (exit 0)
• New + adjacent tests pass:
  • src/common/config/schemas/appConfigOnDisk.test.ts (new flag happy-path + string-rejection)
  • src/node/config.test.ts (round-trip persist on true, drop key on false, ignore invalid)
  • src/node/orpc/router.test.ts (updateAutoHideSidebar + getConfig + on-disk loader)
  • src/browser/hooks/useAutoHideSidebar.test.tsx (8 cases mirroring useChatTranscriptFullWidth)
  • src/browser/features/Settings/Sections/GeneralSection.test.tsx (loads + persists the new toggle)
• Manual smoke (see GIF above): toggle in Settings collapses the sidebar instantly; hover expands smoothly with the existing 200ms transition; leave collapses again; quitting/relaunching restores the persisted state from ~/.mux/config.json; autoHideSidebar: true appears in the JSON when enabled and is removed when disabled.

Risks

Low.

• New persisted key is additive and optional; older clients ignore it (loader uses parseOptionalBoolean, writer uses passthrough-friendly partial object).
• Hover handlers are gated on autoHideEnabled, so when the feature is off the <LeftSidebar> tree is byte-identical to today.
• The persisted sidebarCollapsed value is deliberately preserved while auto-hide is on, so disabling the toggle returns the user to exactly the manual collapse state they had before.

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