Refactor mail sender icons for improved styling consistency (#15)

* Align module/record icons with TYPO3 v14 icon style

The Mail Sender module icon used a solid green rounded-square
background with white glyphs. In the TYPO3 v14 module menu (and the
dark backend sidebar) this clashed with the surrounding monochrome
line icons and looked out of place.

Redesign both the module icon and the sender-address record icon to
follow the official @typo3/icons single-color convention:
- envelope glyph uses `currentColor` so it inherits the menu text
  color and adapts to light/dark backends
- the validation badge uses `var(--icon-color-accent, #ff8700)` so it
  picks up the backend accent color instead of a hard-coded green

This also fixes the record icon's badge ring, which was hard-coded
`white` and therefore invisible on light backgrounds.

* Add colored extension icon for the extension list

The extension had no icon in the Extensions module list because it was
missing the conventional Resources/Public/Icons/Extension.svg file
(the old ext_icon.* location was deprecated in v12 / removed in v13).

Add a colored Extension.svg using the extension's green brand tile with
the same envelope + validation-check motif as the module/record icons,
so it renders as a colored tile in the extension list like other
extensions while staying a coherent visual family.

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Co-authored-by: Claude <noreply@anthropic.com>

Author: Nemo64