A9: right-click context menu (Cut/Copy/Paste/Select All) for all text fields

iced 0.14 ships no native right-click menu on text_input. Add a custom
ContextInput widget (src/widget/context_input.rs) that wraps text_input,
intercepts right-click to read the inner text_input::State selection, and
renders a themed 4-action overlay menu operating on that selection.

- Pure, grapheme-indexed edit seam (copy/cut/paste/select_all over
  iced text_input::Value), unit-tested for ASCII and multi-byte/emoji.
- iced::advanced Widget + overlay::Overlay; clipboard via &mut dyn
  Clipboard, edits published through the existing on_input/on_paste.
- Cut/Copy disabled on empty selection (and on secure fields), Select
  All disabled on empty field, Paste always enabled; dismiss on
  click-out / Esc / item-click.
- Route all 10 text_input call sites in app/mod.rs through context_input.
- Cargo.toml: enable iced "advanced" feature (same crate, no new dep).

459 lib tests (+5), clippy --all-targets clean, release green.

Implemented by Codex (gpt-5.5), senior-audited against the 5-point brief
and re-verified (tests/clippy/release) here.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-Authored-By: Codex (gpt-5.5) <noreply@openai.com>

Cargo.toml

@@ -57,7 +57,7 @@ async-trait = "0.1.89"
 base64 = "0.22.1"
 bytes = "1.11.1"
 dirs = "6.0.0"
-iced = { version = "0.14.0", features = ["canvas", "image", "tokio"] }
+iced = { version = "0.14.0", features = ["advanced", "canvas", "image", "tokio"] }
 # W4 custom avatars: decode/resize an arbitrary user image (png/jpeg only to keep
 # the codec surface small). The matching native file picker (`rfd`) is platform-
 # gated below — its backend differs per OS (xdg-portal on Linux, Win32 on Windows).