fix(offline): exact-size quota eviction and budgeted manifest sync

[QQSHI13]

What

The service worker's quota handling used fake sizes everywhere: 5 MiB assumed per evicted entry, a flat 20 MiB free-space margin, and files were recorded as cached even when they were evicted. On small-quota browsers (Safari, iOS) a large site thrashed: the sync downloaded the whole manifest, hit the quota wall, evicted the just-cached files, and marked them cached anyway — leaving offline coverage at a few tail files that were never re-fetched.

Changes

• cache-manifest.json now ships a sizes map — exact byte count of every built file (docsforge/build.py). Additive; files/version unchanged, so existing SWs and stored previous-files lists stay compatible.
• Measured eviction — makeSpaceIfNeeded accounts each entry at its actual size (Content-Length, falling back to reading the body) and frees until available + freed covers the required bytes plus a proportional 10%-of-quota margin. No more 5 MiB guess, no more 20 MiB floor.
• Budget-aware sync — the manifest sync checks storage.estimate() free space and reserves a flat 20 MiB per download (the usage estimate lags behind in-flight writes). Once the budget is exhausted the sync stops instead of downloading files that will only be evicted again; unbudgeted files are cached on demand when visited. A single resource larger than the whole quota is never cached.
• Truthful tracking — evicted entries are removed from the persisted docsforge-manifest-files list (both during sync and between syncs), so a later sync re-fetches what was actually evicted.

Tests

• Regression: sizes map present, every manifest key covered, values match the built files on disk.
• Integration: sizes matches os.path.getsize for a real build.
• New e2e (Playwright + --quota-override-size-mb): budget test (25 MiB quota → exactly 1 file synced, tracked files all really cached) and eviction test (3 MiB quota, 2×2 MiB assets → LRU evicted with measured accounting, cache within quota, no tracked-but-missing files). Both skip when the override isn't honoured.
• Note: docsforge/templates/assets/javascripts/bundle.min.js was regenerated by build_frontend.py — semantically identical (esbuild identifier-mangling only), fixing a pre-existing committed-vs-built drift so the frontend parity check stays green.

Changelog: 12.5.5 section (en + zh).