refactor: remove dead IPC channels, markup and a volume shim

[BernardJen]

Removes dead code found while writing the ADRs in #346. Nothing in the app behaves differently.

#356: two unused IPC channels

• get-settings-path: its renderer caller went in the Settings redesign (feat: lay the input controls over the wall and rebuild Settings #334).
• diag-log-reset: no caller anywhere in git history.

Both are removed from src/main/index.js and src/preload/index.js.

settings.example.json claimed that "Settings shows the exact path". That stopped being true with #334. The comment now points at the log's startup line, which prints the logs folder that settings.json sits next to.

I didn't write the folder name in. Electron's userData name is unclear: there's no top-level productName, so Electron would use input-viewer, but electron-builder may set productName when it packages the app. labs-videowall-config reports %APPDATA%\input-viewer\ on the wall.

#357: dead markup and a shim

• #bottom-logo (markup, CSS and the element lookup). Single view's bottom logo was removed on 2026-01-16 (e6d918b); since then setLayout hid it in both layouts.
• .input-label (CSS, the two lookups and the two guarded writes). The element left index.html in 77acd52 (2025-12-30), so both lookups were null and the writes never ran. This is wider than the issue listed: the writes in startVideoStream, for mock and real inputs, were dead too.
  • The test fixture (test/helpers/renderer-dom.js) still created .input-label nodes. Its comment said they "mirror index.html", which hasn't been true since then. Both are fixed.
• renderDropdownVolumeControls(): a one-line shim for renderDropdownInputLists(). Its only caller ran straight after a call to that same function, so the pickers rendered twice at startup.

Test plan

• npm test: 58 files, 937 tests pass
• npm run lint clean
• npm run build succeeds
• git grep finds no remaining references to any removed name
• Launch the app: not done locally, because npm 11 blocked Electron's install script so there was no Electron binary. The renderer suite boots renderer.js against a fixture derived from its own getElementById calls, so a stale lookup would have failed there.

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