fix(desktop): polish sidebar and composer interactions

[IchenDEV]

Canonical change Artifacts

• docs/sdlc/changes/2026-08-31-remove-project-disclosure-icon/change.md
• docs/sdlc/changes/2026-08-31-replace-sidebar-drag-with-dnd-kit/change.md
• docs/sdlc/changes/2026-08-31-remove-composer-resize-grip/change.md
• docs/sdlc/changes/2026-08-31-animate-pane-splits/change.md
• docs/sdlc/changes/2026-08-31-align-environment-popover/change.md
• docs/sdlc/changes/2026-08-31-restore-sidebar-session-summary/change.md
• docs/sdlc/changes/2026-08-31-align-sidebar-search-shortcut/change.md
• docs/sdlc/changes/2026-08-31-simplify-provider-model-picker/change.md

All use schema 2 and are verified. Intent and PR creation were approved by the user's direct UI requests and follow-up pr. The highest risk is medium; each Artifact records its exact scope.

Outcome

• Replace native HTML5 sidebar dragging with @dnd-kit/react so projects and tasks can be reordered and moved into sections reliably.
• Restore the sidebar's second line with the latest AI reply, Provider mark, and compact relative age.
• Remove the project disclosure chevron and the nonfunctional Composer resize grip while preserving row and Composer behavior.
• Add direction-aware split-pane entrance motion and align the environment popover and sidebar search shortcut.
• Combine Provider and model selection into one compact T3-inspired picker with a Provider rail and model search.

Verification

• Every acceptance criterion maps to commands or rendered evidence in its canonical Artifact.
• Every Artifact records a verified verdict, verifier, date, and residual risk.
• bun test passed in apps/desktop.
• bun run build:renderer passed lint, TypeScript, and the Vite production build.
• cargo test -p codetwo-core passed all Core unit and integration tests.
• Rendered Browser checks covered physical pointer drag/drop and persistence, second-line summaries, split motion, popover/shortcut geometry, resize-grip removal, and Provider/model search and selection.
• bun test script/verify/checks.test.ts, documentation verification, committed SDLC verification, and worktree SDLC verification passed.

Review and release

• Risk level: medium
• Release target: none
• Rollback: revert commit 198a637c
• Merge approval: pending human review
• Release approval: not applicable
• This PR does not infer merge, deployment, or release authority from implementation work.