breadhelp: add --screenshot CLI mode for automated capture

Three views ("home", "learn", "ask") — one per tab in the Home/Learn/Ask
Stack, switched via the same set_visible_child_name the tab switcher
itself uses, then captured as a full known-size canvas (the window is a
plain top-level, not layer-shell).

Plumbed through breadhelp's own cli::Action/parse() (three more fields,
same shape as force_onboard/autostart/suggest) rather than clap, matching
the app's existing non-clap idiom.

The HANDLES_COMMAND_LINE + thread_local HANDLE singleton architecture
needed one addition beyond the usual NON_UNIQUE-flag fix: every
invocation (including a --screenshot one) normally forwards to the
already-built window over D-Bus and reuses it — worse than the plain
GApplication case in the other apps, since here it's not just "message
the existing instance" but literally switching tabs on and re-capturing
the operator's real, live help-center window. NON_UNIQUE is now set
whenever --screenshot is present in argv (checked before the
Application is even built, since cli::parse() only runs per-invocation
inside connect_command_line).

Cargo.toml

@@ -18,3 +18,7 @@ toml = "0.8"
 toml_edit = "0.22"
 async-channel = "2"
 bread-utils = { git = "https://git.breadway.dev/Breadway/bread-ecosystem", tag = "v0.3.0", features = ["toml"] }
+# Capture primitives for `--screenshot` mode — see src/screenshot.rs. Not
+# tag-pinned like the deps above since it doesn't have a tagged release yet.
+bread-screenshots = { git = "https://git.breadway.dev/Breadway/bread-ecosystem", branch = "dev" }
+anyhow = "1"