fix: make the template workflow real, and the UI legible

The headline flow — fill a config from its template — did not work. Three
bugs, each individually enough to make the tool feel unfinished:

- `add_missing` was unreachable. `find_binding` returned the first match in
  a linear array and `edit_append` was also bound to `a`, 17 entries earlier.
  Bindings now resolve through one table built from the config, and
  `KeybindsConfig::conflicts()` reports any key bound twice instead of
  letting one silently shadow the other. `add_missing` moves to `<Space>`,
  with `A` to take every missing default at once.
- Item status was ignored on write. Every writer fell back to
  `template_value`, so "(missing)" keys landed in the output regardless and
  saving was close to a copy. Unaccepted entries are now omitted.
- Parse errors were swallowed, then the good file was overwritten. A
  malformed target opened as an empty editor and `:w` truncated it. Parse
  failures are now fatal, reported with context, before the TUI starts.

Also:

- Split "differs from the template" (ItemStatus::Overridden) from "you
  edited this" (ConfigItem::dirty). A real config differs from its example
  on nearly every line, so the old Modified flag painted the whole list
  orange and carried no signal.
- Splice template-only entries in at their natural position instead of
  appending them, so a missing nested key no longer strands itself at the
  bottom of the tree at depth 3.
- Preserve .env comments and blank lines across a save.
- Header naming the output file and the template it is compared against,
  plus missing/changed counts. Previously nothing on screen said which
  files were in play or where `:w` would write.
- `?` opens a two-column keybinding reference built from the user's own
  config, scrollable on short terminals. The status bar drops to a few
  contextual hints; the old one joined 11+ hints into an unwrapped line
  that truncated at 80 columns.
- Foldable groups, which the `+ ` prefix had always implied.
- Search covers values as well as keys, reports `match n/m`, and `Esc`
  restores the pre-search cursor.
- Quitting with unsaved changes is refused; added `:q!`, `:x`, `:w <path>`.
- Status messages expire after 4s instead of permanently replacing hints.
- Cancelling a rename on a just-created item removes it again.
- Editing a Number field with text demotes the type instead of failing
  obscurely at write time.
- Undo-tree branches are reachable via `g-`/`g+` and reported on undo.
- Poll instead of blocking, so resizes repaint; drop the unused mouse
  capture that was costing terminal text selection for nothing.
- `--format` accepts xml, ini and properties, which were already supported.

36 tests, up from 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5mrbEFmwv2A4cdzbcSiS1

Cargo.toml

@@ -2,7 +2,7 @@
 authors = ["Nils Pukropp <nils@narl.io>"]
 edition = "2024"
 name = "mould"
-version = "0.5.1"
+version = "0.6.0"
 
 [[bin]]
 name = "mould"