Simplify the pull request reader in the desktop app

The pull request reader repeats itself and has several low-contrast or misleading states.

Section tabs highlight in purple (the location token) instead of the selection colour, and the review sub-tabs use a second underline row that competes with them.
Each section repeats its tab name as a heading.
Unknown check and review states draw a near-invisible ring.
A requested reviewer with no review yet reads as "Unknown review state", wraps, and misaligns its icon.
Check and review status appear three times: header pills, sidebar card and tabs.
The header detaches the PR number from the repository and shows "? → ?" for unknown branches.
Check rows repeat their outcome in the detail line; thread rows show "Resolution unknown · Unknown author".
Threads have three ways back to the list (Replies tab, Back to threads button, Threads tab).
Empty sections are a single muted line.
Reviewers show an initial where the GitHub avatar could be.

Outcome: one place for each piece of status, selection-coloured tabs, readable neutral states, and quieter rows.