🤖 feat: optional flat chat list for the sidebar with project badges

[ibetitsmike]

Summary

Adds an optional flat sidebar mode: a new "Flat chat list" setting replaces the project-folder grouping with one globally sorted list of chats, keeping pinned chats in a single unified block at the top and tagging each root chat card with a project-name badge tinted by the project's folder color. The General settings page is also reorganized from three monolithic headers into logical groups (Appearance, Sidebar, Transcript, Terminal, Archiving, Editor & debugging, Projects).

Background

Users working across many projects lose vertical space and orientation to per-project folders when they mostly care about "all my recent chats". The flat mode drops the folders while badges preserve project attribution; grouped mode stays the default and is unchanged.

Implementation

• sidebarFlatMode persists via usePersistedState (key in src/common/constants/storage.ts, default off) with a command palette toggle for parity with the sibling sidebar options.
• buildSortedWorkspacesFlat (workspaceFiltering.ts) reuses the existing comparators: pinned roots first in global pinnedAt order, then recency with deterministic tie-breakers, then the tree flatten that keeps sub-agents under their parents. Age grouping and hide-sub-agents both compose with flat mode.
• Pinned drag-reorder in flat mode uses one shared reorder group; workspaceService.reorderPinned now scopes its timestamp re-deal to the union of project buckets referenced by the input ids (grouped drags keep exact single-bucket behavior), fixing cross-project reorders reverting on reload.
• Root chat cards and drafts render a truncating badge (resolveSectionColor of the project's configured color); scratch chats get no badge, multi-project chats a neutral one, sub-agent rows none.
• Folder expansion state is left untouched while flat, so toggling back restores the prior grouped layout.

Validation

• Remote dogfood UAT (2 rounds) on dev.coder.com against the exact pushed SHAs: grouped-default regression, live toggle, unified pinned block, badge colors matching folder colors, 240px truncation, sub-agent nesting, hide/age toggles, expansion-state restoration, keyboard nav, drafts, rapid toggling, and cross-project pinned reorder persistence incl. reload (round 2, after the fix).
• Cross-project reorder fix is red-green covered: the new reorderPinned cross-bucket test fails without the service change.

Risks

• reorderPinned is the only backend touch; scope generalization is covered by new unit tests plus the pre-existing single-bucket suite (defensive contract unchanged).
• Grouped sidebar rendering paths are gated behind the (default-off) flag; largest regression surface is ProjectSidebar render wiring, exercised by the updated UI tests and a new Storybook story.

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