build: bundle the fonts the placeholder pane will need

Groundwork for item 30, the blank right pane. Nothing references these
yet; the pane itself is the next piece of work.

The pane is drawn from the user's own HTML mockup, which sets its
wordmark in Oxanium and its text in IBM Plex Sans and pulls both from
Google Fonts. That @import cannot work here: the web view's interceptor
blocks every request by default, deliberately, and neither font is
installed on the development machine either. Shipping them is what
makes the pane look the same on every machine.

Both are SIL OFL 1.1, which permits redistribution and is compatible
with the GPL, and each carries its licence text beside it. The README
records the pairing.

Both are subsets, since they are embedded into the rendered document as
data URIs and the full families would dwarf the page using them.
Oxanium is instanced to weight 800 and cut to the nine characters of
the wordmark: 43K to 1.2K. IBM Plex Sans is cut to Latin-1 plus common
punctuation rather than to the exact strings in use: 525K to 13K. The
wider cut is deliberate, because that font carries interface text that
will change, and a subset matching only today's wording would break
silently the moment a string is edited. Coverage of both was verified
against the strings they have to draw.

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

1 files changed, 28 insertions, 0 deletions

diff --git a/README.md b/README.md
index 060e702..13002d8 100644
--- a/README.md
+++ b/README.md
@@ -449,6 +449,34 @@ strangers, and it is treated that way.
 
 GPL-2.0-only. See `LICENSE` for the full text.
 
+### Bundled fonts
+
+Two fonts are bundled under `assets/fonts/`, both licensed **SIL Open Font
+License 1.1**, which is compatible with the GPL and permits redistribution.
+Each ships with its own licence text beside it.
+
+| Font | Used for | Licence |
+|------|----------|---------|
+| Oxanium ExtraBold | the `qtMailDir` wordmark | `assets/fonts/OFL-Oxanium.txt` |
+| IBM Plex Sans Regular | the placeholder pane's text | `assets/fonts/OFL-IBMPlexSans.txt` |
+
+Both are **subsets**, not the complete fonts, because they are embedded into a
+rendered document as data URIs and the whole family would be far larger than
+the page using it. Oxanium is instanced to weight 800 and cut to the nine
+characters of the wordmark, 43K down to 1.2K. IBM Plex Sans is cut to Latin-1
+plus common punctuation rather than to the exact strings in use, 525K down to
+13K: it carries interface text that will change, and a subset matching only
+today's wording would break the moment a string is edited.
+
+To regenerate either after changing what is drawn:
+
+```bash
+fonttools varLib.instancer Oxanium[wght].ttf wght=800 -o Oxanium-800.ttf
+pyftsubset Oxanium-800.ttf --text="qtMailDir" --flavor=woff2 \
+    --layout-features='' --desubroutinize \
+    --output-file=assets/fonts/Oxanium-ExtraBold-subset.woff2
+```
+
 ## Development Approach
 
 This project is developed using AI-assisted tools. Code is generated with the help of AI based on human-provided specifications, design decisions, and iterative feedback.