cli: suggest the closest flag or command on a typo

An unknown --flag now logs the nearest accepted spelling within two
edits as did_you_mean, painted green next to the red typo in the pretty
log format; logfmt stays plain. A bare first argument within two edits
of a command name is rejected with the same hint instead of being
fetched as a url.

The Levenshtein helper moves from SlashCommand into string.zig so the
agent REPL and the CLI share it, with the table widened to fit the
longest flag name.

src/agent/SlashCommand.zig

@@ -23,6 +23,7 @@
 
 const std = @import("std");
 const lp = @import("lightpanda");
+const string = @import("../string.zig");
 const Command = lp.Command;
 const Config = lp.Config;
 
@@ -108,31 +109,5 @@ pub const all_names: [browser_tools.names.len + meta_commands.len + llm_values.l
 
 /// Closest command name within two edits, or null — for "did you mean?" on typos.
 pub fn closestCommand(name: []const u8) ?[]const u8 {
-    var best: ?[]const u8 = null;
-    var best_dist: usize = std.math.maxInt(usize);
-    for (all_names) |cand| {
-        const dist = editDistance(name, cand);
-        if (dist < best_dist) {
-            best_dist = dist;
-            best = cand;
-        }
-    }
-    return if (best_dist <= 2) best else null;
-}
-
-/// Case-insensitive Levenshtein distance. Returns `maxInt` for inputs longer
-/// than the table (no slash command is that long).
-fn editDistance(a: []const u8, b: []const u8) usize {
-    const max = 32;
-    if (a.len >= max or b.len >= max) return std.math.maxInt(usize);
-    var dp: [max][max]usize = undefined;
-    for (0..a.len + 1) |i| dp[i][0] = i;
-    for (0..b.len + 1) |j| dp[0][j] = j;
-    for (a, 1..) |ca, i| {
-        for (b, 1..) |cb, j| {
-            const cost: usize = if (std.ascii.toLower(ca) == std.ascii.toLower(cb)) 0 else 1;
-            dp[i][j] = @min(@min(dp[i - 1][j] + 1, dp[i][j - 1] + 1), dp[i - 1][j - 1] + cost);
-        }
-    }
-    return dp[a.len][b.len];
+    return string.closest(name, &all_names, 2);
 }