[codex] Improve project-grouped sidebar

[sunheyi6]

Summary

• Add a clickable status/project grouping toggle with pressed-button semantics.
• Render project-grouped sessions as folder sections with an initial four-session preview and a Show more control.
• Let each project folder header expand or collapse only its own conversations.

Why

The project grouping view previously did not match the requested folder-style design, and project headers were not interactive. This makes the sidebar behave like a project tree while preserving the compact session rows.

Validation

• npm --workspace @maka/ui run build
• npm --workspace @maka/desktop run build:main
• node --test dist\main\tests\session-project-view-contract.test.js
• node --test dist\main\tests\sidebar-scroll-contract.test.js
• npm --workspace @maka/desktop run build:renderer

@Astro-Han please take a look.

[Astro-Han]

Reviewed #557 and applied fixes on top — branch fix/557-collapse-grid-ui-unify, rebased onto latest main (incl. #571/#572).

Findings & fixes:

1. Collapsed sidebar layout regression. The 5-row grid was kept in collapsed mode even though the view-mode toggle is display:none there. A display:none item doesn't participate in grid layout, so the footer dropped into the minmax(0,1fr) track and the settings button floated mid-rail. Fix: collapsed mode uses a 4-row grid; added a CSS contract test.
2. Project group headers + "show more" used raw <button>, diverging from the status-group header which already routes through UiButton (variant=quiet, size=nav) for the shared hover/focus/active contract. Migrated both to UiButton; dropped the bespoke :hover/:focus-visible/svg CSS. Also fixed content centering — UiButton's base adds justify-center, which centered the heading under width:100%; set justify-content: flex-start.
3. Session rows over-indented under projects. The 32px (--space-8) left indent was redundant (folder hierarchy is already conveyed by the heading icon + group spacing) and pushed rows ~2 chars in from the heading. Removed so rows align left.
4. Test hardening. Folder-header disclosure assertion is now attribute-order independent; view-mode pressed assertion updated for SettingsSegmented (fix(ui): designer-audit round 1 — sidebar overlap, count mismatch, tab split, empty-state placement, vaporware chrome #571 rerouted the toggle through it).

Validation: @maka/ui + @maka/desktop:main build clean; both sidebar contract suites pass (12/12); typecheck clean.

Note: the original view-mode toggle CSS had hardcoded px + referenced non-existent tokens — #571 already replaced it with SettingsSegmented, so this branch adopts #571's version.