refactor(desktop): unify settings page rhythm and layout - #236

Outcome

Settings pages no longer each carry their own layout dialect. One measure, one page-title block, one
section-heading role and weight, one module-card plane, and one 28px button treatment now apply to
every tab:

• the 64rem profile/worktrees overrides are gone; every tab renders in the 48rem .settings-page;
• Appearance's six hand-rolled sections and Memory's unstyled h2/h3 render through the shared
  SettingsSection/GroupHeading, and both shared headings now carry the same 14px/600 treatment
  (rendered verification caught SettingsSection at 500 against GroupHeading and PageHeader at
  600 and the fix is included);
• Usage renders its title through PageHeader instead of a hand-rolled h1;
• worktree, memory, and profile module cards moved from fill-quiet to the documented surface
  plane, with the memory policy columns separated by a hairline instead of a second fill;
• size="sm" is now byte-identical to size="compact", so the 28px control has one look.

Change record

• Change: docs/sdlc/changes/2026-09-16-unify-settings-page-rhythm/intent.md

The linked change directory owns scope, authorization, acceptance, evidence, and residual risk.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected: all nine
  settings tabs were driven in the real renderer and measured at 768px, and the UI Lab scenario
  captured the shared section anatomy in light and dark (see the record's AC-1..AC-7 evidence).
• Required checks passed: bun run lint, bunx tsc --noEmit, bun test (940 pass / 3 skip /
  0 fail), bun run build:renderer, bun script/verify/sdlc.ts --worktree,
  bun script/verify/docs.ts, and git diff --check.
• Merge authorization: pending (this PR was requested; merge is not authorized)
• Release authorization: not requested