feat(community): cancel a pending join to withdraw + remove it

[stormer78]

Problem

A Pending community (from a join request) could not be deleted, archived, or left — those actions are all gated to inactive states, and Pending is neither. The only escape was the 7-day client timeout. A user who joined the wrong community (or hit a stale DIDComm session that leaves the join stuck Pending) had no way to back out and retry.

What this does

Adds a cancel pending join action:

• c on a Pending row (y/n confirmed) → transitions the membership to a new terminal CommunityStatus::Withdrawn and tears down its now-dead DIDComm session.
• Withdrawn is inactive (archive/delete eligible) and re-joinable, and — like Left — a voluntary outcome that never raises the actions-required badge.
• The user can then press d to delete it, or just re-join. Mirrors the existing leave → delete pattern.

VTC notification is best-effort (for now)

vta-sdk — including 0.17, the version the library/binary actually build against (the 0.12 git pin is test-only via vta-service) — has no applicant-side withdraw DIDComm message, only the withdrawn status the VTC reports in a status-response. So the cancel is fully local for now; the request also lapses to the VTC's own timeout.

• The inbound status-response handler now reconciles a VTC-reported withdrawn to the same Withdrawn state.
• Real VTC notification is a follow-up: it needs a join-requests/withdraw/1.0 message added to vta-sdk in the VTI repo first (the protocol layer lives there), then wired at the TODO(VTI) in the WithdrawJoin action.

Changes (8 files)

• openvtc-core/src/config/account.rs — Withdrawn variant; is_inactive() / needs_attention() updates; guarded withdraw() transition (Pending-only).
• openvtc-core/src/messaging.rs — inbound "withdrawn" reconciliation.
• openvtc/src/state_handler/... — Action::{CommunityConfirmWithdraw, CommunityCancelWithdraw, WithdrawJoin}, arm/disarm nav handling, and the async WithdrawJoin handler (best-effort notify + transition + session teardown).
• openvtc/src/state_handler/main_page/... — is_pending summary field, Withdrawn status label.
• openvtc/src/ui/pages/main/... — c-key gating (pending-only), confirm prompt, help-text c: cancel.

Tests

New unit tests for: the withdraw() transition + guards, status classification, the withdrawn serde tag, inbound status-response reconciliation, and the c-key gating + confirm flow.

cargo build, cargo test, and cargo clippy all green (one pre-existing clippy warning at mod.rs:138, untouched).

Follow-up

• VTI: add join-requests/withdraw/1.0 to vta-sdk so cancel can actively notify the VTC, then wire the send here (see TODO(VTI)).