refactor: clean up and professionalize the codebase

  Structured error handling:
  - Replace catch-all Error::WgTui(String) with 7 typed variants:
    InvalidInterfaceName, TunnelAlreadyExists, Config, CommandFailed,
    Validation, Import, NoTunnelsToExport
  - Migrate all 27 call sites in wireguard.rs to appropriate variants

  DRY config parsing:
  - Extract WgConfig parser struct with parse(), interface_value(),
    all_values(), interface_has_any_key() to replace 4 functions that
    duplicated INI-section parsing boilerplate
  - Optimize add_server_peer() to parse config once instead of 5 times

  Stronger path types:
  - Change import_tunnel, import_zip_conflict_count, import_tunnels,
    export_tunnels_to_zip from &str to &Path parameters
  - Move expand_path() calls to call sites in app layer
  - Extract open_conf_archive() helper to deduplicate zip validation

  Split monolithic app.rs (1412 lines) into focused modules:
  - app.rs: App struct, AppFlags bitfield, core state management
  - app/input.rs: event handling, all consume_* handlers
  - app/render.rs: draw() and all render_* methods
  - app/wizard.rs: wizard types and supporting structs

  Consolidate duplicated patterns:
  - Extract InputAction enum + handle_text_input() helper to eliminate
    7x duplicated Backspace/Char/Esc match arms across input handlers
  - Define WizardSteps trait implemented by both Client and Server
    wizards, replacing manual 5-method delegation
  - Add NewTunnelWizard::create() to collapse duplicated completion
    branches from 24 lines to 8
  - Unify interface name validation into single shared helper

Author: excoffierleonard

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "wg-tui"
-version = "0.2.1"
+version = "0.3.0"
 edition = "2024"
 description = "A terminal user interface for managing WireGuard VPN tunnels"
 repository = "https://github.com/excoffierleonard/wg-tui"
@@ -22,11 +22,7 @@ ratatui = "0.30.0"
 shellexpand = "3.1.1"
 thiserror = "2.0.18"
 which = "8.0.0"
-zip = { version = "7.2.0", default-features = false, features = ["deflate"] }
+zip = { version = "7.4.0", default-features = false, features = ["deflate"] }
 
 [lints.clippy]
-pedantic = { level = "warn", priority = -1 }
-cargo = { level = "warn", priority = -1 }
-# Transitive dependencies pull in multiple versions of common crates (syn, bitflags, thiserror, etc.)
-# which we cannot control directly
-multiple_crate_versions = "allow"
+pedantic = "warn"