feat(theme): add Gaia Aether palette

[ykzird]

Summary

Adds a ninth built-in theme, Gaia Aether, to the Theme enum: Gruvbox
warmth (cream #f2e5bc text, moss and amber hues) on a blue-grey #181c22
surface, with a dusty-mauve accent. It matches the AETHER / G.A.I.A v2
desktop palette I already use everywhere else, so a document read in this
viewer sits in the same colours as the rest of the desktop.

Follows the documented extension path in src/theme/mod.rs — one enum
variant, one Tokens constructor in src/theme/themes.rs, one dispatch arm
in src/theme/tokens.rs — so nothing else needed touching. The settings
modal, config parsing, and the syntect mapping pick it up automatically.

Changes

• src/theme/mod.rs — Theme::GaiaAether variant, Theme::ALL, label()
  ("Gaia Aether"), and syntax_theme_name()
  ("base16-eighties.dark", the warm dark bucket shared with Dracula/Gruvbox Dark).
• src/theme/themes.rs — gaia_aether(), with the source-palette names
  carried through as comments.
• src/theme/tokens.rs — dispatch arm.
• README.md — theme count updated to nine, new bullet in the Themes list,
  gaia_aether added to the config.toml example.
• CHANGELOG.md — ## [Unreleased] entry.

Three slots deliberately depart from the source palette, because the literal
values fail this crate's existing contrast audit — the details are in the
gaia_aether() doc comment and the changelog entry:

Slot	Source value	Used	Why
accent.primary	#a0607a	#c98fa0	No foreground reaches AA on #a0607a (ceiling 3.76:1); it carries on_accent text in the status and tab bars. Source value stays as accent.alt.
state.selection_bg	#261a28	#503a4a	Source sits 0.16 luminance units from the surface, under MIN_SELECTION_DELTA (3.0) — an invisible selection.
state.focus	—	#c98fa0	6.45:1 against the base surface.

Test Plan

• Existing tests pass (cargo test --workspace) — 1621 pass, 3 ignored, 0 fail, on Linux
• New tests added where appropriate — none needed; the audit tests in
  src/theme/mod.rs::tests and src/theme/tokens.rs::tests iterate
  Theme::ALL, so the new variant is pinned to the same WCAG AA floor, focus
  visibility, and selection-distinctness checks as every other theme
• Manually tested on macOS / Linux (specify which) — Linux only (Arch, rustc 1.98.1); macOS untested
• If a Mermaid block changed, verified rendering in TUI and via mermaid-text CLI — no Mermaid change

Also run locally, per CONTRIBUTING.md:

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets -- -D warnings — clean

Manual check: launched the viewer against this repo's README.md, opened the
settings modal (c), confirmed Gaia Aether appears as the ninth entry and
applies live, and compared the rendered output against the desktop palette
(dark blue-grey surface, cream reading text, mauve selection highlight, moss
and cyan headings).

Checklist

• CHANGELOG.md entry added (root for markdown-tui-explorer, crate-local for mermaid-text)
• No new clippy warnings (cargo clippy --all-targets -- -D warnings)
• Documentation updated if public API changed — README theme section; no public API change
• Version bumped in Cargo.toml if user-visible — left to your release commit, since your release flow bumps the version in a separate release: commit and I did not want to guess the next number

Linked Issue

None — no issue for this; happy to open one first if you would rather review
it as an issue than as a PR.