feat(sidebar): group inactive chats by project

[0xSolarPunk]

Before

• In project-and-activity mode, inactive chats appeared in one flat cross-project section.
• Folder context was repeated on each inactive row, without project-level collapse or reorder boundaries.

flowchart LR
    chats[Chats] --> active[Active project groups]
    chats --> inactive[Inactive section]
    inactive --> flat[Flat mixed project list]
    chats --> archived[Archived flat list]

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After

• Inactive chats now sit under Inactive and are grouped by project or folder.
• Active and inactive project collapse state is independent, nested project grouping is retained, and reorder scopes stay within each inactive project.
• Archived chats remain flat under Archived.

flowchart LR
    chats[Chats] --> classify{Activity}
    classify --> active[Active project groups]
    classify --> inactive[Inactive section]
    inactive --> folders[Project or folder groups]
    classify --> archived[Archived flat list]

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The row model now applies the existing project grouping context inside the Inactive section, with section-specific collapse keys and reorder scopes. Deterministic before and after screenshots document the visible behavior.

Files

• Sidebar row model and virtual-row contracts
• Sidebar collapse wiring and project header behavior
• Regression coverage for grouping, nesting, collapse, sorting, and search transitions
• Deterministic before and after screenshots

Tests

• Focused sidebar suites: 93 passed
• bun run check: passed; 0 Svelte errors and two existing warnings
• bun run start --port 0: production build completed before the smoke timeout
• git diff --check, impact, and scope checks: passed
• bun run test: limited by the existing reader-worker v9 test timing out; no sidebar failure observed

Prompted by: yk (@chiayong)

[0xSolarPunk]

Closing this fork-local PR because its base branch is behind upstream main and expanded the diff beyond this feature. The feature branch will be reopened against cfal/garcon:main.