Add local-node log viewer to the dashboard

[alytaphoenix]

Summary

• Adds a "Logs" page to the dashboard for viewing this node's application logs (main process log + runner stdout/stderr) without needing terminal/SSH access.
• New backend endpoints: GET /v1/logs (list), GET /v1/logs/{name} (tail, JSON), GET /v1/logs/{name}/raw (full file download), modeled on the existing traces endpoints.
• name is restricted to a fixed whitelist (main / runner_stdout / runner_stderr) mapped to known paths — never joined with client input, so there's no path traversal surface.
• Tail reads at most 2MB from the end of the file (_tail_file), so large/unbounded logs don't get loaded in full.
• Local-node only by design (no cross-node aggregation) — exo is distributed and each node writes its own log; aggregating logs across a cluster would be a separate, much larger feature.

Test plan

• uv run basedpyright — 0 new errors (baseline unchanged at 319 pre-existing errors from unrelated mlx import resolution issues in this sandbox)
• uv run ruff check / ruff format — clean
• uv run pytest src/exo/api/tests/test_logs_api.py — 10/10 passing, including a regression test for a lines<=0 tail-length edge case caught during review
• uv run pytest src/exo (excluding suites that require the mlx package, unavailable in this sandbox and failing identically on main) — 331 passed, 3 skipped
• npx svelte-check — no new errors (baseline 15 errors / 6 warnings unchanged)
• Manually verified in a real browser (Playwright): nav link, log selector, tail content rendering, and file download all work end-to-end against a running exo instance
• nix fmt / nix flake check were not run (nix isn't available in this sandbox) — substituted ruff format + prettier-plugin-svelte directly, which should match, but this hasn't been confirmed against the real treefmt config

🤖 Generated with Claude Code

https://claude.ai/code/session_011rjSfwDBTkmySmfU6NgHKF