fix(desktop): polish TaskBoard session workspace

[IchenDEV]

Canonical change Artifact

• Task-to-Session workspace polish
• Follow-up to merged PR feat(desktop): make tasks a session workspace #214

Outcome

• Preserve destination Session drafts and cancel stale inspector prompt insertion after navigation.
• Render historical, loading, failed, and beyond-cap PR states truthfully with labelled status indicators.
• Keep the Inspector as a flat, distinct surface panel on wide layouts and switch list/detail in place on narrow layouts—no drawer, overlay, or scrim.
• Align the shared titlebar and strengthen the light surface tone against the white canvas.

Verification

• Focused TaskBoard/theme tests: 64 passed, 259 assertions.
• Full desktop suite: 839 passed, 0 failed, 5,005 assertions.
• TypeScript, ESLint, Stylelint, and bun run build passed.
• Docs, SDLC, worktree SDLC, and proposed-file diff hygiene passed.
• Live browser checks passed at 1440×900 and 760×900, including no horizontal overflow and focus restoration.

Review and release

Risk is medium because composer state and primary TaskBoard navigation changed. This PR is Draft for human review. Merge and release are not authorized.