Scroll the whole file sideways, not each row by its own amount

Sharing one `ScrollState` across the viewer's rows was not enough to make
them move together. `Modifier.horizontalScroll` is a node per row, and each
one coerces the shared offset into *its own* range -- its content width
less its viewport -- so a short line's range is zero and it stayed put
while the long line beside it moved. Each row also writes `maxValue` on the
shared state as it measures, so how far the file could be dragged at all
was decided by whichever row happened to measure last, and changed as the
list scrolled.

Both go away once every row is the same width. `FileLines` now carries the
longest line in columns, and the viewer turns that into one content width
from a single character's advance -- arithmetic rather than twenty thousand
measurements, because the face is monospace -- and gives it to every row. A
tab counts as eight columns and deliberately upwards: over-estimating
leaves a little empty space past the longest line, under-estimating puts
the end of that line out of reach. The width is capped well under what
`Constraints` can carry, so a minified file is a scroll that stops early
rather than a crash.

Reported by Iris on 2026-09-04. Checked on the emulator against the
generated 1 MB file, whose lines run from one character to sixty-eight:
the file now moves as a block, the offset survives scrolling vertically and
newly composed rows arrive at it, and the far end of the longest line is
reachable. Also checked on the two cases the change had no reason to touch
-- a file narrower than the screen, which still does not scroll at all, and
an empty one, whose zero content width draws its one numbered line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/androidApp/src/test/kotlin/com/example/aiapp/FileLinesTest.kt

@@ -54,6 +54,20 @@ class FileLinesTest {
         }
     }
 
+    /**
+     * The number every row in the viewer is sized to. It has to be the widest line, because rows of
+     * their natural widths scroll sideways by different amounts -- see [FileViewer].
+     */
+    @Test
+    fun `the column count is the widest line, counting a tab as eight`() {
+        assertEquals(5, FileLines.of("one\nthree\nx\n", null).columns)
+        // A tab counts up to eight, and upwards on purpose: over-estimating leaves empty space
+        // past the longest line, under-estimating puts its end out of reach.
+        assertEquals(9, FileLines.of("\tx\nshort\n", null).columns)
+        // An empty file is one empty line, which is no columns at all rather than an error.
+        assertEquals(0, FileLines.of("", null).columns)
+    }
+
     @Test
     fun `a file with no language is plain`() {
         val lines = FileLines.of("fn main() {}\n", null)