Centre the empty state and widen its primary button

Splits the converter screen into a fixed header and a body region that
behaves differently per state. The empty state centres its label and button
on both axes in whatever space is left; the working states keep scrolling,
since format pickers and progress can exceed the screen and centring content
that overflows would push it out of reach.

The primary button now fills the width and stands 56dp tall rather than the
Material default of 40dp, so it reads as the main affordance instead of a
small control adrift in an otherwise empty screen. Horizontal screen padding
drops to 16dp while vertical stays at 24dp, which is what lets a full-width
button sit close to both edges.

Both dimensions are named constants rather than inline numbers, because the
same treatment is applied to the primary action in every other state.

No theme changes were needed. Dark mode already tracked the device: the
Compose theme reads isSystemInDarkTheme() and the activity theme has a
values-night variant. Verified on an API 37 emulator by toggling
`cmd uimode night` and capturing both -- mean luminance 245/255 in light
against 18/255 in dark, with the button recolouring and status bar icons
inverting correctly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.idea/.name

@@ -1 +0,0 @@
-Android(MediaConverter

.idea/gradle.xml

@@ -1,5 +1,6 @@
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.idea/misc.xml

@@ -1,7 +1,7 @@
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-  <component name="ProjectRootManager" version="2" languageLevel="JDK_25" project-jdk-name="jbr-25" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_25" default="true" project-jdk-name="temurin-25" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/build/classes" />
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