[Cleanup] Remove dead client variable, use Buffer.indexOf for newline counting, document source maps

[LarsLaskowski]

📖 Description

Bundles three independent, minor housekeeping items from the 2026-07 security & performance review:

1. src/client/main.ts — removed the dead displayedLines variable in the SSE log-line handler. It was declared but never assigned, always null, and passed into completeSseTiming's perf-timing details. The real displayed-line count is already recorded separately by the render-side render:log-lines timing, since the actual render is deferred to a requestAnimationFrame batch.
2. src/server/logTailer.ts — replaced the byte-by-byte for…of loop counting \n bytes in readLastLines with Buffer.prototype.indexOf, which performs the same scan natively instead of one JS iteration per byte. This path can scan up to MAX_TAIL_SCAN_BYTES (8 MB) on bootstrap, reattach, and skip-ahead. Semantics are unchanged (0x0a never appears as a UTF-8 continuation byte, per the existing comment, which is preserved).
3. README.md — added a short note documenting that client source maps are built and served intentionally (to ease in-browser debugging; the source is public anyway), and that the server-side map is not exposed since express.static only serves dist/client.

None of these changes alter user-visible behavior.

🎫 Issues

Closes #139

👩‍💻 Reviewer Notes

The readLastLines change is the one with the most surface area — worth a close look at src/server/logTailer.ts. The new regression tests exercise CRLF input, a file without a trailing newline, and a file spanning multiple 64 KB read chunks with zero newlines, to confirm the indexOf-based scan behaves identically to the removed byte loop.

📑 Test Plan

• Added src/server/logTailer.test.ts cases:
  • normalizes CRLF line endings the same way as LF on bootstrap
  • returns the trailing line even when the file does not end with a newline
  • counts newlines correctly across multiple 64 KB read chunks (regression for #139)
• Ran npm test (all 155 tests pass) and npm run typecheck.
• Ran npm run build to confirm both bundles build cleanly.

✅ Checklist

General

• I have added unit tests for my changes, per UNIT_TESTS.md.
• I have run npm test and npm run typecheck locally and they pass.
• I have updated the project documentation (README, ARCHITECTURE, etc.) to reflect my changes, where applicable.
• I have read the CONTRIBUTING documentation and followed the project's code style and conventions.

Server-specific (src/server)

• I have added or updated a module in src/server (tailer, parser, buffer, routes, SSE hub, config, etc.).
• I have kept src/server/types.ts in sync with src/client/state.ts for any shared payload change (no shared payload change in this PR).
• I have preserved existing server-side bounds and defaults (buffer/SSE limits, poll interval clamps) unless the change explicitly targets them.

Client-specific (src/client)

• I have added or updated a client module (render.ts, main.ts, derivedLogView.ts, filters.ts, preferences.ts, etc.).
• I have kept the frontend framework-free and added any new default UI state to createInitialState() in src/client/state.ts (no new state introduced).
• I have kept every physical log line as its own visible UI row (continuation lines are not merged).

⏭ Next Steps

None.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud