bos-settings: full, non-destructive control of every bread* config

The bread/breadpad/breadcrumbs/breadbox views wrote invented schemas
(e.g. top-level log_level, [[profile]] name/ssids) that did not match the
apps' real TOML, so they showed empty and — worse — clobbered the real
config on Save, since the old config::save serialized only the keys it
modelled.

Rework the config layer onto toml_edit: parse each file into a
DocumentMut, mutate only the specific keys a view exposes, and write it
back preserving comments and any unmodelled keys (calendar password,
saved-network passwords, model paths). Unit-tested.

Add ui/widgets.rs (switch/entry/password/dropdown/spin/float/csv rows +
view scaffold + save button) bound to the shared document, then rewrite
the four views against the real schemas with far more coverage:

- bread: [daemon], [lua], [modules], all five [adapters.*] with their
  sub-options, [events], [notifications]
- breadpad: [settings], [model] + [model.ollama], [reminders], [calendar]
- breadcrumbs: [settings] (7 keys), [[networks]] editor, [profiles.*] editor
- breadbox: fixed to real [[contexts]] name/priority array editor

Goal: configure everything from the GUI rather than hand-editing TOML.

bos-settings/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "bos-settings"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 
 [dependencies]
@@ -9,4 +9,8 @@ glib = "0.20"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "0.8"
+# toml_edit drives non-destructive config editing: it preserves comments and
+# any keys the UI doesn't model, so saving a single field never rewrites or
+# drops the rest of the user's config file.
+toml_edit = "0.22"
 async-channel = "2"