feat(convert): --force to overwrite existing contracts; commas in messages

[rizukirr]

Summary

Two changes to apic convert, plus a message-style sweep.

apic convert --postman --force

• Adds a --force flag that overwrites contracts that already exist.
• The default is unchanged (it still errors on an existing file), and the error
  now suggests the flag:

$ apic convert --postman MPOS.postman.json
Error: .../verify_otp.json already exists, pass --force to overwrite

$ apic convert --postman MPOS.postman.json --force   # overwrites

Threaded through convert::run / write_contracts; the GUI import keeps the
safe default (never overwrites).

Message style: commas over semicolons and em-dashes

Swept user-facing runtime messages (errors, warnings, status lines, TUI/GUI
labels, and apic read's response-title line) to use commas instead of ; or
—, e.g.:

• already exists; refusing to overwrite -> already exists, pass --force to overwrite
• No project template found; create will use... -> ..., create will use...
• unsupported by apic — imported as GET -> unsupported by apic, imported as GET
• RESPONSE 200 — OK -> RESPONSE 200, OK

Scope is runtime message string literals; clap --help doc comments and code
comments were left alone (say the word if you want those swept too).

Tests

Added an overwrite case to the write_contracts unit test. cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings are clean;
cargo test --workspace passes (258 tests). Smoke-tested the CLI end-to-end
(first import succeeds, re-import errors with the --force hint, --force
overwrites).