fix(notes): explain the locked last chip and fix the one-note pool copy

[wolasss]

Whittling the note pool down to a single note left two rough edges in "Which notes":

• The guarantee line read "Shuffled — you get all 1 before any repeats.", which is
  nonsense for a one-note pool. It now reads "One note — it repeats until you add
  another." The multi-note copy is unchanged.
• Clicking the last selected chip did nothing. The settings reducer deliberately refuses
  to empty the pool, but nothing in the UI said so, so it read as a dead tap. The sole
  remaining chip now carries aria-disabled and a title explaining that the last note
  stays selected, plus a subdued not-allowed style in src/index.css.

The chip keeps its onClick and stays in the tab order rather than taking a real
disabled attribute, so keyboard and screen-reader users can still land on it and hear
why it won't toggle. The reducer guard itself is untouched — it's the behaviour being
explained.

Verification

npm run check (lint, e2e typecheck, vitest, build) passes — 437 tests across 37 files.
New src/components/NotePoolCard.test.tsx covers the singular copy, the multi-note copy
and the locked chip (including that unselected chips get neither attribute); the existing
App.integration.test.tsx pool-guarantee assertions stay green.

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Live preview (this branch): https://agents.marten-gecko.ts.net:15511/