fix(ui): render ANSI pages on a black backdrop (2.0.4)

[felipedbene]

Fixes the readability bug an F-Droid reviewer found while testing the v2.0.3 reproducible build (fdroiddata!41663):

the ANSI viewer paints the fixed xterm palette from AnsiParser.BASIC onto the Material surface, so on the light theme white and bright grey text (argb(229,229,229), argb(255,255,255)) is white on near white. In the colour CTA map the station labels were effectively invisible; the same file is fine in dark mode.

What changed

TextViewer now gives ANSI content the background its colours were chosen against:

• black backdrop when AnsiParser.hasAnsi(text);
• uncoloured runs default to xterm colour 7 (0xFFE5E5E5) instead of Material onSurface, which would be dark-on-black.

Plain text pages are untouched and still follow the theme.

Forcing a dark backdrop rather than remapping the light end of the palette: remapping changes the colours the art's author picked, and needs separate handling for the 256-colour and truecolour paths. Pure black rather than a dark grey because it matches the palette's own colour 0, so ESC[40m fills blend instead of showing as boxes.

Verification

:app:assembleDebug and :app:testDebugUnitTest pass. Installed on a device forced to the light theme (cmd uimode night no) and opened gopher://gopher.debene.dev/0/map.ansi — the station labels the reviewer called out (SKO, EVN, WRG, LPZ, OPK, UNC, WIL, SOX, MDW) render legibly, colours preserved.

Release

Bumps to 2.0.4 / versionCode 4 with a changelogs/4.txt entry. After merge the F-Droid recipe needs Builds[].commit pinned to the merged SHA, a v2.0.4 tag to publish the reference APK, and an update to the fdroiddata MR.

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