fix: no grey strip under the terminal's last row (#6)

[Maxaubert]

Closes #6.

The bug

Owner screenshot on the Pitch (black) theme: a grey bar along the bottom of the terminal.

xterm sizes itself in whole rows. MEASURED in the app: a 604px terminal inside a 611px box (15px at the owner's scale). The pixels under the last row belonged to a transparent element, so what showed there was the native window background, not the theme. The 4px frame round the terminal was painted; that leftover strip was not.

The fix

The panel paints the ground and xterm's canvas is clear:

• TerminalPanel's box is bg-[var(--p-bg)] with 4px of padding (it was a transparent box with a --p-bg border).
• currentTermTheme() hands xterm background: #00000000, plus a named cursorAccent: it defaults to the background, which on a clear canvas would make the character under a block cursor a hole. Checked on the page: the cursor draws its character in the theme's ground on the cursor colour.
• Every pixel of the panel (rows, frame, leftover strip) gets exactly ONE coat, which is also what acrylic needs, and --p-bg already carries the window's alpha, so the acrylic path needs no special case any more.

Verification

• The e2e theme scenario now MEASURES it: it finds the strip under the last row, walks up from the pixel to the first painted ground, and compares it with --p-bg-solid, in the dark default and again after switching to a light theme.
  • Against the OLD code: FAIL and it is the theme's ground (none vs 11,11,15) (nothing was painted there).
  • With the fix: 11,11,15 vs 11,11,15, and 244,244,244 after the switch.
• npm run typecheck: clean. npm run lint: 0 errors. npm test: 265 pass. npm run e2e: 10 scenarios pass.
• Installed on this machine as a local combination of this branch and Agent indicators follow the theme's accent; minimal by default (#4) #5 (so the indicator changes under test there were not taken away); exe timestamp polled (23:18:08 -> 08:06:56). That combined branch is local only and is not part of this PR.

Notes

• Merges cleanly with Agent indicators follow the theme's accent; minimal by default (#4) #5 (tried locally; both touch CLAUDE.md and tools/e2e/run.mjs in different places).
• The grey bar you may still see on the RIGHT while the pointer is over the terminal is xterm's own auto-hiding scrollbar; it is invisible once the pointer leaves (checked: invisible, opacity 0). Unchanged by this PR.
• Unsigned. Version stays 0.1.0 (nothing released yet).

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