feat(terminal): add the YourSSH Dark color theme

[thangnm93]

Summary

Adds a 45th terminal color scheme, "YourSSH Dark", as the app's own and places it first in the catalog. Neutral charcoal base, deliberately desaturated ANSI set so long log output stays flat and readable, azure/teal accents on the cursor, selection and current search hit.

Closes #105.

Changes

• _yourSshDark in app/lib/theme/terminal_themes.dart — all 23 fields (background/foreground/cursor/selection, 8 normal + 8 bright ANSI, 3 search-hit colors), registered as kTerminalThemes[0]. No other theme is touched.
• Index 0 also makes it the fallback terminalThemeByName resolves to for an unknown name (e.g. a theme id arriving from sync that this build does not have). SettingsProvider's stored default stays Dracula, so no existing install changes appearance on upgrade.
• Tests: catalog count 44 → 45, plus YourSSH Dark leads the catalog and is the fallback theme. The pre-existing invariants (semi-transparent selection per [Bug UI] Highlight/Selection color is too dark and unreadable in the HOST terminal tab #40, unique names) now cover the new entry too — its selection is #2E4A57 at AA.
• README feature count and screenshot table; CHANGELOG entry under [Unreleased] → Added.
• Screenshot harness: feature_screenshots_test.dart now opens the theme picker dialog and captures it (05-settings/05-terminal-theme-picker.png), closing it with Esc so the following scroll steps still hit the settings scroll view.

Palette

role	hex		ANSI	normal	bright
background	#1A1D20		black	#1F2225	#4E545C
foreground	#C9CCD1		red	#CF6A6A	#E08A8A
cursor	#56B6D8		green	#7CB07A	#97C795
selection	#2E4A57 @ AA		yellow	#D6A15C	#E8BC7A
searchHitBackground	#D6A15C		blue	#5A9BD4	#7DB4E3
searchHitBackgroundCurrent	#5A9BD4		magenta	#A98BC9	#C2A6DD
searchHitForeground	#1A1D20		cyan	#5FB3B8	#7FCBCF
			white	#B8BCC2	#E8EAED

Type of change

• feat — new feature
• fix — bug fix
• refactor / polish — no behavior change
• docs — documentation only
• test — tests only
• release — version release to master

How was this tested?

• cd app && flutter analyze — no new warnings
• cd app && flutter test — all tests pass (1722)
• Manually verified on: macOS

Beyond the widget tests, the theme was checked against a live local shell driven by an integration test and verified by sampling rendered pixels, not by eye — background and all six sampled ANSI colors match the palette exactly (magenta reads #A88AC8 on some samples from text antialiasing).

Screenshots

The picker with the new entry first, and the "45 themes" counter:

Checklist

• PR title follows Conventional Commits
• Code and comments are written in English
• New/changed behavior is covered by tests
• No secrets, credentials, or personal data in the diff

Note for the reviewer

develop's README/CHANGELOG do not yet carry these two doc lines, so this PR adds them. The same two lines were also swept into release/0.2.0's commit 14fdbed (they were sitting uncommitted in the tree when that release commit was made), so expect a trivial same-content conflict when release/0.2.0 merges back into develop.