feat(frontend): compact board and review spacing

[Hardik180704]

Closes #2882

Hey @Pulkit7070 and @codebanditssss, this tightens up the spacing across the board and review surfaces without changing their behavior.

What changed:

• made board headers and session cards more compact
• reduced spacing in the Summary and Reviews panels
• tightened the files toolbar, file rows, and diff container
• switched inspector tabs to accessible icon-only controls on narrow rails so they do not overflow
• added focused spacing assertions to the existing component tests

Checked with:

• npm run typecheck
• npm run typecheck:e2e
• npx vitest run — 1,038 tests passed
• CI=true npm run test:e2e:renderer — 20 tests passed
• Prettier and git diff --check
• visual checks at normal and narrow desktop widths
• Electron desktop launch

[Pulkit7070]

Reviewed. This is a clean, behavior-preserving spacing pass and it is the right PR for #2882 (that issue is still open; nothing else has done this work).

What I checked:

• The changes are padding/gap/height class reductions only (h-13 to h-11, py-2 to py-1, gap-3 to gap-2, px-4 to px-3, min-h-11 to min-h-10, diff min-height 220 to 180, and a single tokens.css value). No logic or DOM-structure changes, so behavior is unaffected.
• The icon-only inspector tabs on narrow rails are done accessibly: the label span is hidden with @max-[350px]/inspector:hidden, but you added aria-label and title on the button, so the accessible name is preserved when the text is not shown. That is the correct pattern for icon-only controls.
• CI is green here (test, format, scan, renderer-smoke), which now includes the typecheck gate on main, so the full vitest suite and typecheck pass.

One coordination note, not a defect. This touches surfaces that other open PRs also edit, so expect merge conflicts depending on order:

• SessionInspector.tsx and SessionInspector.test.tsx also change in #3053 (two-section Reviews tab). That is the highest-overlap one, since both restructure the inspector tabs/Reviews area. Whichever lands second will need a rebase.
• SessionsBoard.tsx overlaps #2997 (scrollbars) and tokens.css is touched by a few PRs.

No behavioral issues found. The spacing values themselves are a visual judgment call best confirmed at normal and narrow widths, which the description says was done.

[Hardik180704]

Visual verification for #2882 — compact board cards after the spacing pass. Statuses, titles, branches, and PR details remain visible while the cards use less vertical space.

[Pulkit7070]

Approving. Behavior-preserving spacing changes only, the icon-only inspector tabs keep their accessible name via aria-label and title, and CI is green including the typecheck gate on main. Note that this and #3053 both edit SessionInspector.tsx, so whichever merges second will need a rebase.