feat: add a config option to set the default view mode to hide closed issues on launch

[MichaelSims]

The problem

After some time, closed issues build up in the default view mode. The v view-mode cycle (All -> Active -> Ready) hides them, but it resets to All on every launch, so I have to press v every session. There's no config option for the startup default.

Desired behavior

Add a config key that controls the initial view mode at startup:

• Key view.mode (string), accepted values all | active | ready, default all (preserves current behavior)
• On launch, App.viewMode is initialized from this key instead of always defaulting to ViewModeAll
• The in-app v / V cycle keeps working and can still move away from the configured default for the current session
• An unknown/invalid value falls back to all (optionally with a warning)

Acceptance criteria

• With view.mode: active in ~/.abacus/config.yaml, launching abacus starts in the Active view closed/blocked/deferred hidden) with no v press
• With view.mode: ready, it starts in the Ready view
• With the key absent or all, behavior is unchanged from current
• An invalid value does not crash; it falls back to all
• v / V still cycle view modes at runtime as before

Out of scope

• Persisting a v-chosen mode back to config (making the current setting sticky across launches). This request is just for a read-only startup default