web: clickable repo tree + per-file blob viewer

[tlongwell-block]

web: clickable repo tree + per-file blob viewer (text / markdown / image / binary)

Closes the gap raised in the proj-git-on-s3 channel: the SPA cloned each repo into IndexedDB but had no way to open an individual file. This adds the missing route + viewer with no JS execution (security non-goal) and no relay-side changes.

What

• New route /repos/$repoId/blob/$ (splat path) → RepoBlobPage. Adds an entry to web/src/app/routes.ts; tanstack-router regenerates routeTree.gen.ts.
• New hook useGitBlob returning a discriminated BlobView union (text | markdown | image | binary | too-large). Backed by readBlobView() in git-client.ts, which reuses the existing isomorphic-git IndexedDB clone.
• New shared hook useRepoContext(repoId) lifting the {owner, repoName, defaultRef} resolution that the detail page does inline. Consumed by RepoBlobPage.
• New viewer RepoBlobViewer.tsx (~255 LOC): markdown via the existing react-markdown + remarkGfm; text via a plain <pre>; raster images via <img src=blob:> with the object-URL created/revoked in a component effect (never cached); binary + over-cap → "Download" only.
• Tree rows in RepoTreeSection.tsx: blobs become <Link> to the new route; trees stay visibly muted/non-clickable (sub-tree navigation deferred — keeps the v1 promise honest).

Security non-goals (load-bearing)

• No JS execution. Same-origin would put pushed JS next to the user's session — a real ask, not solved here. That's option 3 ("Sprout Pages" on a separate sandboxed origin), a separate project.
• SVG is rendered as text, never as <img>. SVG can carry active content. The smoke test pushes an SVG containing <script>alert(…)</script> and confirms it renders as visible text and the alert never fires.

Preview caps

• TEXT_PREVIEW_LIMIT_BYTES = 1 MiB — over → "too-large" + Download. (A 1 MiB string is already big to render in the DOM.)
• IMAGE_PREVIEW_LIMIT_BYTES = 10 MiB — over → same. Lets normal-sized PNGs preview inline without surprise rejection.
• Binary: no preview cap. Always download.

Order: image-by-extension first (so a 2 MiB PNG isn't accidentally rejected by the text cap), then binary detection (NUL byte sniff), then text/markdown.

Files

 web/src/app/routeTree.gen.ts                  |  25 ++-     (generated)
 web/src/app/routes.ts                         |   1 +
 web/src/app/routes/repos.$repoId.blob.$.tsx   |   6 +       (new)
 web/src/features/repos/git-client.ts          |  97 +++++   (added readBlobView + caps)
 web/src/features/repos/ui/RepoBlobViewer.tsx  | 255 +++++   (new)
 web/src/features/repos/ui/RepoDetailPage.tsx  |   9 +-      (pass repoId to tree)
 web/src/features/repos/ui/RepoTreeSection.tsx |  38 ++--    (links + muted dirs)
 web/src/features/repos/use-git-browse.ts      |   6 +-      (useGitBlob → readBlobView)
 web/src/features/repos/use-repo-context.ts    |  35 ++++    (new)
 web/tests/e2e/smoke-blob.mjs                  |  58 ++++    (standalone smoke driver)

10 files, +512 / −18. No new dependencies. No relay changes.

Verification

Local relay (just relay-web against block/sprout @ 9a10e62, with a separately-fixed h1/h2 cred workaround documented in WORK_LOGS), then vite dev pointed at ws://localhost:3000, then headless chromium drove the matrix:

Fixture	Expected kind	Result
hello.txt	text	text-viewer with Copy button
README.md	markdown	rendered via react-markdown + remarkGfm
pixel.png (1×1 transparent)	image	<img> from object-URL, Download button
diagram.svg with <script>	text	rendered as literal text; alert never fires
big.bin 1.43 MiB	binary	"Binary file — 1.43 MiB", Download only
dir with spaces/odd#name.txt	text	splat decodes correctly via params._splat

Build green: pnpm typecheck, pnpm exec biome check ., pnpm build.

Reviewed by Max (GPT-5.5) at plan + implementation stages, integrating all correctness nits before this PR opened.