feat(format): expand pow_format modifiers

Extend the pow_format spec parser beyond alignment so configs can shape the
rendered number, not just pad it. A forced scale (#, k, M, G, T, P) pins the SI
prefix instead of auto-selecting it and, since the author then knows both scale
and unit, hides the prefix and unit by default; U brings the unit back, u hides
it in auto mode, and i forces integer display. b and B override the decimal or
binary base independently of the call site, and a trailing width now fixes the
coefficient field when a scale is forced, overflowing to '#' when it does not
fit.

The old '>' and '<' branches rendered via fmt::format("{}", s), which discarded
every new field on the recursive call; they now build from a single render path
that reads the current formatter, so modifiers survive alignment. The auto path
is byte-identical to before, verified against the previous implementation.

Adds a Catch2 suite covering each modifier alone and combined, plus the
backward-compatible alignment cases, and documents the modifiers in the network
and disk man pages.

man/waybar-disk.5.scd

@@ -146,6 +146,29 @@ Addressed by *disk*
 
 *{specific_free}*: Amount of available disk space for normal users in a specific unit. Defaults to bytes.
 
+# NUMBER FORMAT MODIFIERS
+
+*{total}*, *{used}* and *{free}* auto-scale with a binary prefix (KiB, GiB, …).
+Their rendering can be tuned with fmt-style modifiers, e.g. *"{free:>}"* or
+*"{free:G}"*, combined in any order:
+
+*<*, *=*, *>*: Alignment/padding (left, column-align, right).
+
+*u* / *U*: Hide (*u*) or show (*U*) the unit suffix. Shown by default with an auto
+scale, hidden by default when a scale is forced.
+
+*#*, *k*, *M*, *G*, *T*, *P*: Force a fixed scale instead of auto-selecting
+(*#* = base scale). Forcing a scale hides the scale prefix and, by default, the
+unit. E.g. *"{free:G}"* always shows gibibytes.
+
+*i*: Force integer display (no decimals).
+
+*b* / *B*: Force decimal base 1000 (*b*) or binary base 1024 (*B*); disk values
+are binary by default.
+
+A trailing number is a fixed width for the coefficient when a scale is forced;
+overflow is shown as *#* characters (e.g. *"{free:=3#}"* → *###*).
+
 # EXAMPLES
 
 ```