fix: build a file's diff editor only when it nears the viewport

[omartelo]

Opening Files changed on a wide pull request locked the app (#92). Every file
in a diff panel starts expanded, and an expanded file builds its own CodeMirror
view — so #91's 191 files were 191 EditorViews constructed in a single commit,
each followed by a lazy language chunk re-configuring its document. The existing
LARGE_FILE_LINES guard is evaluated per file and never fired: not one of the
191 was large, the diff was wide.

What changed

• FileDiff defers each editor to an IntersectionObserver. A card holds a
  height-estimated placeholder until it comes within 600px of the viewport. The
  placeholder has to carry the file's height, or every card would sit in the
  viewport at once and nothing would be deferred. Mount is one-way, so scrolling
  back keeps a view's selection and highlighting; collapsing the file still
  destroys it. This is the shared path, so Pulls' Files changed and the dock's
  Review tab both get it — the dock had the same root cause.
• The Pulls diff stops refetching on window focus. HEAD moving already
  refetches it; replaying a wide diff on every alt-tab back into lich was the
  most surprising half of the bug.

Measured

The frontend suite runs in node with no jsdom, so none of this is reachable by
the gate. Measured by hand instead: #91's real diff (191 files, 10,020 lines)
rendered through the same FileDiff cards, headless Chromium at 1600x1000.

	editors at rest	main thread to idle
before	191	13,678 ms
after	8	465 ms

Scrolling to the bottom builds 11 editors in total, and every visible card
carries its own. LINE_HEIGHT_PX = 17 is calibrated against the diff theme's
measured 16.8px line box.

Not in this PR

From the issue's list, deliberately skipped — with 465 ms remounts they buy
nothing today:

• caching the parsed diff by (path, head)
• budgeting how many files start expanded per panel
• the backend ceiling at internal/project/pr.go:325 (427 KB was never the
  bottleneck)

Test plan

• Gate run from node_modules/.bin rather than through pnpm, which
  summarises biome's output and reported a false green here: biome check .
  clean, tsc --noEmit clean, 406 tests, vite build ok. After merging
  main: go test ./... green and gofmt -l . empty.
• Editor count and time-to-idle measured before/after on refactor(frontend): adopt biome, dedupe the repeated code, group lib/ by domain #91's diff
• Hand pass in the app: this pull request is reviewed and merged from lich's
  own Files changed tab — the screen the fix lands on.

Closes #92

[omartelo]

Second commit on the branch: the two Loading… lines on this screen are now skeletons.

Both waits are gh round-trips of about a second — the pull request lookup covers the whole screen, the diff covers the Files changed tab — and a single muted line on that much empty space read as a failure rather than as work in progress. Each now holds the shape of what follows: title, actions, status line and tabs for the pull request; file tree, toolbar and file cards for Files changed. The tree placeholder honours the panel's own hidden/shown preference, so nothing jumps when the diff lands.

The primitive is shadcn's skeleton, with one adaptation: stock fills with bg-muted, which in this palette is the same value as the hover fill — invisible in light, near-invisible in dark once the pulse halves it. It fills with a foreground opacity step instead, which reads in both themes. Verified by rendering both skeletons headless in light and dark.